Delay check for prototype on __fastcall functions until after MergeFunctionDecl.

In C, it is only known after merging decls if a function with 0 arguments has
a prototype.  Fixes PR20386, see that for more notes.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214408 91177308-0d34-0410-b5e6-96231b3b80d8