[OpenCL] Fix pipe built-in functions return type.

By default return type of call expressions calling built-in
functions is set to bool.

Fixes https://llvm.org/bugs/show_bug.cgi?id=30219.

Reviewers: Anastasia

Subscribers: dmitry, cfe-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D24136



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280800 91177308-0d34-0410-b5e6-96231b3b80d8