[Clang] Make '-Wglobal-constructors` work on the GNU attributes (#129917)

Summary:
The `-Wglobal-constructors` option is useful for restricting the usage
of global constructors / destructors. However, it currently ignores the
attributes that introduce global constructors, meaning that the module
can still have ctors if `-Werror` is set. If this is intentional by the
user, I believe it would be more correct to push the diagnostic.

Author: jhuber6

clang/docs/ReleaseNotes.rst

@@ -62,6 +62,9 @@ AST Dumping Potentially Breaking Changes
 Clang Frontend Potentially Breaking Changes
 -------------------------------------------
 
+- The ``-Wglobal-constructors`` flag now applies to ``[[gnu::constructor]]`` and
+  ``[[gnu::destructor]]`` attributes.
+
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2138,6 +2138,8 @@ static void handleConstructorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (AL.getNumArgs() &&
       !S.checkUInt32Argument(AL, AL.getArgAsExpr(0), priority))
     return;
+  S.Diag(D->getLocation(), diag::warn_global_constructor)
+      << D->getSourceRange();
 
   D->addAttr(::new (S.Context) ConstructorAttr(S.Context, AL, priority));
 }
@@ -2147,6 +2149,7 @@ static void handleDestructorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (AL.getNumArgs() &&
       !S.checkUInt32Argument(AL, AL.getArgAsExpr(0), priority))
     return;
+  S.Diag(D->getLocation(), diag::warn_global_destructor) << D->getSourceRange();
 
   D->addAttr(::new (S.Context) DestructorAttr(S.Context, AL, priority));
 }

clang/test/SemaCXX/attr-require-constant-initialization.cpp

@@ -337,6 +337,11 @@ constexpr TestCtor<NotC> inval_constexpr(42); // expected-error {{must be initia
 ATTR constexpr TestCtor<NotC> inval_constexpr2(42); // expected-error {{must be initialized by a constant expression}}
 // expected-note@-1 {{in call to 'TestCtor(42)'}}
 
+[[gnu::constructor]] void ctor() {}
+// expected-warning@-1 {{declaration requires a global constructor}}
+[[gnu::destructor]] void dtor() {}
+// expected-warning@-1 {{declaration requires a global destructor}}
+
 #elif defined(TEST_THREE)
 #if defined(__cplusplus)
 #error This test requires C