[Clang] Implement CWG2351 `void{}` (#78060)

Per [CWG2351](https://wg21.link/CWG2351), allow `void{}`, treated the
same as `void()`: a prvalue expression of type `void` that performs no
initializat

[Clang] Implement CWG2351 `void{}` (#78060)

Per [CWG2351](https://wg21.link/CWG2351), allow `void{}`, treated the
same as `void()`: a prvalue expression of type `void` that performs no
initialization.

Note that the AST for the expression `T{}` looks like:

```
// using T = int;
CXXFunctionalCastExpr 'T':'int' functional cast to T <NoOp>
`-InitListExpr 'T':'int'
// using T = const int;
CXXFunctionalCastExpr 'int' functional cast to T <NoOp>
`-InitListExpr 'int'
// using T = void;
CXXFunctionalCastExpr 'T':'void' functional cast to T <ToVoid>
`-InitListExpr 'void'
// using T = const void;
CXXFunctionalCastExpr 'void' functional cast to T <ToVoid>
`-InitListExpr 'void'
```

As for `void()`/`T() [T = const void]`, that looked like
`CXXScalarValueInitExpr 'void'` and is unchanged after this.

For reference, C++98 [5.2.3p2] says:

> The expression `T()`, where `T` is a simple-type-specifier (7.1.5.2)
for a non-array complete object type or the (possibly cv-qualified) void
type, creates an rvalue of the specified type, whose value is determined
by default-initialization (8.5; no initialization is done for the
`void()` case). [*Note:* if `T` is a non-class type that is
*cv-qualified*, the `cv-qualifiers` are ignored when determining the
type of the resulting rvalue (3.10). ]

Though it is a bit of a misnomer that, for `T = void`,
`CXXScalarValueInitExpr` does not perform value initialization, it would
be a breaking change to change the AST node for `void()`, so I simply
reworded the doc comment.

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Allow parameter pack expansions and initializer lists in annotate attribute

These changes make the Clang parser recognize expression parameter pack
expansion and initializer lists in attribute argum

Allow parameter pack expansions and initializer lists in annotate attribute

These changes make the Clang parser recognize expression parameter pack
expansion and initializer lists in attribute arguments. Because
expression parameter pack expansion requires additional handling while
creating and instantiating templates, the support for them must be
explicitly supported through the AcceptsExprPack flag.

Handling expression pack expansions may require a delay to when the
arguments of an attribute are correctly populated. To this end,
attributes that are set to accept these - through setting the
AcceptsExprPack flag - will automatically have an additional variadic
expression argument member named DelayedArgs. This member is not
exposed the same way other arguments are but is set through the new
CreateWithDelayedArgs creator function generated for applicable
attributes.

To illustrate how to implement support for expression pack expansion
support, clang::annotate is made to support pack expansions. This is
done by making handleAnnotationAttr delay setting the actual attribute
arguments until after template instantiation if it was unable to
populate the arguments due to dependencies in the parsed expressions.

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Diagnose -Wunused-value based on CFG reachability

(This relands 59337263ab45d7657e and makes sure comma operator
diagnostics are suppressed in a SFINAE context.)

While at it, add the diagnosis mes

Diagnose -Wunused-value based on CFG reachability

(This relands 59337263ab45d7657e and makes sure comma operator
diagnostics are suppressed in a SFINAE context.)

While at it, add the diagnosis message "left operand of comma operator has no effect" (used by GCC) for comma operator.

This also makes Clang diagnose in the constant evaluation context which aligns with GCC/MSVC behavior. (https://godbolt.org/z/7zxb8Tx96)

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D103938

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Revert "Diagnose -Wunused-value based on CFG reachability"

This reverts commit cbbf2e8c8ae7730ff0121f4868de4a7d188feb65.
It seems causing diagnoses in SFINAE context.

Diagnose -Wunused-value based on CFG reachability

While at it, add the diagnosis message "left operand of comma operator has no effect" (used by GCC) for comma operator.

This also makes Clang diagn

Diagnose -Wunused-value based on CFG reachability

While at it, add the diagnosis message "left operand of comma operator has no effect" (used by GCC) for comma operator.

This also makes Clang diagnose in the constant evaluation context which aligns with GCC/MSVC behavior. (https://godbolt.org/z/7zxb8Tx96)

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D103938

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Revert "Diagnose -Wunused-value based on CFG reachability"

This reverts commit 63e0d038fc20c894a3d541effa1bc2b1fdea37b9.

It causes test failures:

http://lab.llvm.org:8011/#/builders/119/builds/561

Revert "Diagnose -Wunused-value based on CFG reachability"

This reverts commit 63e0d038fc20c894a3d541effa1bc2b1fdea37b9.

It causes test failures:

http://lab.llvm.org:8011/#/builders/119/builds/5612
https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8835548361443044001/+/u/clang/test/stdout

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Diagnose -Wunused-value based on CFG reachability

While at it, add the diagnosis message "left operand of comma operator has no effect" (used by GCC) for comma operator.

This also makes Clang diagn

Diagnose -Wunused-value based on CFG reachability

While at it, add the diagnosis message "left operand of comma operator has no effect" (used by GCC) for comma operator.

This also makes Clang diagnose in the constant evaluation context which aligns with GCC/MSVC behavior. (https://godbolt.org/z/7zxb8Tx96)

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D103938

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[Annotation] Allows annotation to carry some additional constant arguments.

This allows using annotation in a much more contexts than it currently has.
especially when annotation with template or co

[Annotation] Allows annotation to carry some additional constant arguments.

This allows using annotation in a much more contexts than it currently has.
especially when annotation with template or constexpr.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D88645

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