[Clang] Make '-Wglobal-constructors` work on the GNU attributes

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/lib/Sema/SemaDeclAttr.cpp

@@ -7596,6 +7596,17 @@ void Sema::ProcessDeclAttributeList(
     D->setInvalidDecl();
   }
 
+  // Warn on global constructors and destructors created by attributes.
+  if (D->hasAttr<ConstructorAttr>() &&
+      !getDiagnostics().isIgnored(diag::warn_global_constructor,
+                                  D->getLocation()))
+    Diag(D->getLocation(), diag::warn_global_constructor)
+        << D->getSourceRange();
+  if (D->hasAttr<DestructorAttr>() &&
+      !getDiagnostics().isIgnored(diag::warn_global_destructor,
+                                  D->getLocation()))
+    Diag(D->getLocation(), diag::warn_global_destructor) << D->getSourceRange();
+
   // Do this check after processing D's attributes because the attribute
   // objc_method_family can change whether the given method is in the init
   // family, and it can be applied after objc_designated_initializer. This is a

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