Don't report "main" as missing a prototype in freestanding mode

[void]

Summary

A user may want to use freestanding mode with the standard "main" entry
point. It's not useful to warn about a missing prototype as it's not
typical to have a prototype for "main".

[aaron.ballman]

LGTM!

clang/lib/Sema/SemaDecl.cpp

ShouldWarnAboutMissingPrototype(const FunctionDecl *FD,
   if (!FD->isGlobal())
     return false;
 
   // Don't warn about C++ member functions.
   if (isa<CXXMethodDecl>(FD))
     return false;
 
   // Don't warn about 'main'.
-  if (FD->isMain())
+  if (isa<TranslationUnitDecl>(FD->getDeclContext()->getRedeclContext()))
+    if (IdentifierInfo *II = FD->getIdentifier())
+      if (II->isStr("main"))
         return false;
 
   // Don't warn about inline functions.
   if (FD->isInlined())
     return false;
 
   // Don't warn about function templates.
   if (FD->getDescribedFunctionTemplate())
     return false;

clang/test/Sema/no-warn-missing-prototype.c

+// RUN: %clang_cc1 -fsyntax-only -Wmissing-prototypes-x c -ffreestanding -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wmissing-prototypes -x c++ -ffreestanding -verify %s
+// expected-no-diagnostics
+int main() {
+  return 0;
+}