[clang][ExprConst] Allow non-literal types in C++23 (#100062)

Instead of diagnosing non-literal types in C++23, allow them and later
diagnose them differently, e.g. because they have a non-constexp

[clang][ExprConst] Allow non-literal types in C++23 (#100062)

Instead of diagnosing non-literal types in C++23, allow them and later
diagnose them differently, e.g. because they have a non-constexpr
constructor, destructor, etc.

For this test:
```c++
struct NonLiteral {
NonLiteral() {}
};
constexpr int foo() {
NonLiteral L;
return 1;
}

// static_assert(foo() == 1);
```
The current diagnostics with c++20/c++23 are:
```console
~/code/llvm-project/build » clang -c array.cpp -std=c++20
array.cpp:91:14: error: variable of non-literal type 'NonLiteral' cannot be defined in a constexpr function before C++23
91 | NonLiteral L;
| ^
array.cpp:87:8: note: 'NonLiteral' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors
87 | struct NonLiteral {
| ^
1 error generated.
------------------------------------------------------------
~/code/llvm-project/build » clang -c array.cpp -std=c++23
(no output)
```

With the `static_assert` enabled, compiling with `-std=c++23` prints:
```console
array.cpp:95:15: error: static assertion expression is not an integral constant expression
95 | static_assert(foo() == 1);
| ^~~~~~~~~~
array.cpp:91:14: note: non-literal type 'NonLiteral' cannot be used in a constant expression
91 | NonLiteral L;
| ^
array.cpp:95:15: note: in call to 'foo()'
95 | static_assert(foo() == 1);
| ^~~~~
1 error generated.
```

As mentioned in #60311, this is confusing. The output with c++20
suggests that using c++23 will make the problem go away, but it's
diagnosed the same when running the function.

With this commit, the output instead diagnoses _why_ the non-literal
type can't be used:
```console
array.cpp:95:15: error: static assertion expression is not an integral constant expression
95 | static_assert(foo() == 1);
| ^~~~~~~~~~
array.cpp:91:14: note: non-constexpr constructor 'NonLiteral' cannot be used in a constant expression
91 | NonLiteral L;
| ^
array.cpp:95:15: note: in call to 'foo()'
95 | static_assert(foo() == 1);
| ^~~~~
array.cpp:88:3: note: declared here
88 | NonLiteral() {}
| ^
1 error generated.

```




Fixes #60311

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Reapply "[Clang][C++23] Implement P2448R2 ..." (#85136) (#85145)

This reverts commit 003e292f9895a9cf4e30688269efa668d1fcbb09 because
there were dependent changes in the codebase that now fail.

Revert "[Clang][C++23] Implement P2448R2 ..." (#85136)

Revert "[Clang][C++23] Implement P2448R2: Relaxing some constexpr
restrictions (#77753)"

This reverts commit 99500e8c08a4d941acb8a7eb005232

Revert "[Clang][C++23] Implement P2448R2 ..." (#85136)

Revert "[Clang][C++23] Implement P2448R2: Relaxing some constexpr
restrictions (#77753)"

This reverts commit 99500e8c08a4d941acb8a7eb00523296fb2acf7a because it
causes a behavior change for std=c++20. See
https://github.com/llvm/llvm-project/pull/77753.

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[Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (#77753)

Per
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html
function/constructor/destructor can be mar

[Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (#77753)

Per
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html
function/constructor/destructor can be marked `constexpr` even though it
never produces a constant expression.
Non-literal types as return types and parameter types of functions
marked `constexpr` are also allowed.
Since this is not a DR, the diagnostic messages are still preserved for
C++ standards older than C++23.

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[clang] Use -std=c++23 instead of -std=c++2b

During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.

This updates the reference to c++2b to c++

[clang] Use -std=c++23 instead of -std=c++2b

During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.

This updates the reference to c++2b to c++23 and updates the __cplusplus
macro.

Drive-by fixes c++1z -> c++17 and c++2a -> c++20 when seen.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D149553

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[Clang] Permit static constexpr variables in constexpr functions

This implement the C++23 paper P2647R1 (adopted in Kona)

Reviewed By: #clang-language-wg, erichkeane

Differential Revision: https:/

[Clang] Permit static constexpr variables in constexpr functions

This implement the C++23 paper P2647R1 (adopted in Kona)

Reviewed By: #clang-language-wg, erichkeane

Differential Revision: https://reviews.llvm.org/D138851

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[Clang] Add a compatibiliy warning for non-literals in constexpr.

Reviewed By: aaron.ballman, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D122249

[Clang][C++2b] P2242R3: Non-literal variables [...] in constexpr

Allow goto, labelled statements as well as `static`, `thread_local`, and
non-literal variables in `constexpr` functions.

As specifie

[Clang][C++2b] P2242R3: Non-literal variables [...] in constexpr

Allow goto, labelled statements as well as `static`, `thread_local`, and
non-literal variables in `constexpr` functions.

As specified. for all of the above (except labelled statements) constant
evaluation of the construct still fails.

For `constexpr` bodies, the proposal is implemented with diagnostics as
a language extension in older language modes. For determination of
whether a lambda body satisfies the requirements for a constexpr
function, the proposal is implemented only in C++2b mode to retain the
semantics of older modes for programs conforming to them.

Reviewed By: aaron.ballman, hubert.reinterpretcast, erichkeane

Differential Revision: https://reviews.llvm.org/D111400

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