Move the no-prototype calling conv check after decl merging

Now we don't warn on this code:
  void __stdcall f(void);
  void __stdcall f();

My previous commit regressed this functionality because I didn't update
the relevant test case which used a definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221188 91177308-0d34-0410-b5e6-96231b3b80d8