[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
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6916710..3392806 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1749,7 +1749,7 @@
                                            Loc, Loc, II, R, /*TInfo=*/nullptr,
                                            SC_Extern,
                                            false,
-                                           /*hasPrototype=*/true);
+                                           R->isFunctionProtoType());
   New->setImplicit();
 
   // Create Decl objects for each parameter, adding them to the