[OpenCL] Add builtin header test

Add a test to verify OpenCL builtin declarations using
OpenCLBuiltins.td.

This test consists of parsing a 60k line generated input file.  The
entire test takes about 60s with a debug build on a decent machine.
Admittedly this is not the fastest test, but doesn't seem excessive
compared to other tests in clang/test/Headers (with one of the tests
taking 85s for example).

RFC: https://lists.llvm.org/pipermail/cfe-dev/2021-April/067973.html

Differential Revision: https://reviews.llvm.org/D97869