[clang] Pass the NamedDecl* instead of the DeclarationName into many diagnostics.

Background:
-----------
There are two related argument types which can be sent into a diagnostic to
display the name

[clang] Pass the NamedDecl* instead of the DeclarationName into many diagnostics.

Background:
-----------
There are two related argument types which can be sent into a diagnostic to
display the name of an entity: DeclarationName (ak_declarationname) or
NamedDecl* (ak_nameddecl) (there is also ak_identifierinfo for
IdentifierInfo*, but we are not concerned with it here).

A DeclarationName in a diagnostic will just be streamed to the output,
which will directly result in a call to DeclarationName::print.

A NamedDecl* in a diagnostic will also ultimately result in a call to
DeclarationName::print, but with two customisation points along the way:

The first customisation point is NamedDecl::getNameForDiagnostic which is
overloaded by FunctionDecl, ClassTemplateSpecializationDecl and
VarTemplateSpecializationDecl to print the template arguments, if any.

The second customisation point is NamedDecl::printName. By default it just
streams the stored DeclarationName into the output but it can be customised
to provide a user-friendly name for an entity. It is currently overloaded by
DecompositionDecl and MSGuidDecl.

What this patch does:
---------------------
For many diagnostics a DeclarationName is used instead of the NamedDecl*.
This bypasses the two customisation points mentioned above. This patches fix
this for diagnostics in Sema.cpp, SemaCast.cpp, SemaChecking.cpp, SemaDecl.cpp,
SemaDeclAttr.cpp, SemaDecl.cpp, SemaOverload.cpp and SemaStmt.cpp.

I have only modified diagnostics where I could construct a test-case which
demonstrates that the change is appropriate (either with this patch or the next
one).

Reviewed By: erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D84656

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[clang][NFC] Add tests for the use of NamedDecl::getDeclName in the unused/unneeded diagnostics.

Consider hidden decls for isUsed checks.

This fixes pr17624.

A FIXME from Richard Smith:

It seems to me that the root cause is that a per-Decl 'used' flag doesn't
really make much sense in the way

Consider hidden decls for isUsed checks.

This fixes pr17624.

A FIXME from Richard Smith:

It seems to me that the root cause is that a per-Decl 'used' flag doesn't
really make much sense in the way we use it now. I think we should either track
whether that particular declaration is used (with isUsed scanning the entire
redecl chain), or we should only have one flag for the entire redeclaration
chain (perhaps by always looking at the flag on either the most recent decl or
the canonical decl). Modeling it as "is this declaration or any previous
declaration used" is weird, and requires contortions like the loop at the end
of Sema::MarkFunctionReferenced.

llvm-svn: 193202

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Mark all subsequent decls used.

In the source

static void f();
static void f();
template<typename T>
static void g() {
f();
}
static void f() {
}
void h() {
g<int>();
}

t

Mark all subsequent decls used.

In the source

static void f();
static void f();
template<typename T>
static void g() {
f();
}
static void f() {
}
void h() {
g<int>();
}

the call to f refers to the second decl, but it is only marked used at the end
of the translation unit during instantiation, after the third f decl has been
linked in.

With this patch we mark all subsequent decls used, so that it is easy to check
if a symbol is used or not.

llvm-svn: 171888

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Fix a regression from the previous commit.
Template instantiation can set the canonical decl to used after subsequent
decls have been chained, so we have to check that too.

llvm-svn: 171088

Use the most recent redecl to decide if it is needed.

This fixes pr14691, which I think is a regression from r168519.

llvm-svn: 171077