Don't diagnose no-prototype callee-cleanup function definitions

We already have a warning on the call sites of code like this:
  void f() { }
  void g() { f(1, 2, 3); }
t.c:2:21: warning: too many arguments in call to 'f'

We can limit ourselves to diagnosing unprototyped forward declarations
of f to cut down on noise.

llvm-svn: 221184