[OpenCL][Sema] Improve BuildResolvedCallExpr handling of builtins

Summary:
This is a follow-up on https://reviews.llvm.org/D52879, addressing a few issues.

This:
 - adds a FIXME for later improvement for specific builtins: I previously have only checked OpenCL ones and ensured tests cover those.
 - fixed the CallExpr type.



Reviewers: riccibruno

Reviewed By: riccibruno

Subscribers: yaxunl, Anastasia, kristina, svenvh, cfe-commits

Differential Revision: https://reviews.llvm.org/D55136

llvm-svn: 348120