[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

llvm-svn: 280800