[Sema] Check if a builtin is FunctionPrototype().

Don't assume it's always is. This prevents a crash in Sema while
trying to merge return type for a builtin w/out function prototype.
 
PR:		23086
Differential Revision:	http://reviews.llvm.org/D9235
Reviewed by:	rsmith

llvm-svn: 235806
2 files changed