Revert "Disable -Wmissing-prototypes for internal linkage functions that aren't explicitly marked "static""

Regresses:
typedef struct {
  static void  f() {
  }
} a_t;

Causing this to error instead of warn, because the linkage is computed
earlier/too early perhaps. I'll send out a review to see if there's some
other path forward or if this is an acceptable regression, etc.

This reverts commit 275c562.

Author: dwblaikie

clang/lib/Sema/SemaDecl.cpp

@@ -14210,9 +14210,6 @@ ShouldWarnAboutMissingPrototype(const FunctionDecl *FD,
   if (!FD->isGlobal())
     return false;
 
-  if (!FD->isExternallyVisible())
-    return false;
-
   // Don't warn about C++ member functions.
   if (isa<CXXMethodDecl>(FD))
     return false;

clang/test/SemaCXX/warn-missing-prototypes.cpp

@@ -44,16 +44,3 @@ void j() = delete;
 extern void k() {} // expected-warning {{no previous prototype for function 'k'}}
 // expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
 // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:{{.*}}-[[@LINE-2]]:{{.*}}}:"{{.*}}"
-
-namespace {
-struct anon { };
-}
-
-// No warning because this has internal linkage despite not being declared
-// explicitly 'static', owing to the internal linkage parameter.
-void l(anon) {
-}
-
-void *operator new(decltype(sizeof(3)) size, const anon &) throw() {
-  return nullptr;
-}