[OpenCL] Add OpenCL builtin test generator

Add a new clang-tblgen flag `-gen-clang-opencl-builtin-tests` that
generates a .cl file containing calls to every builtin function
defined in the .td input.

This patch does not add any use of the new flag yet, so the only way
to obtain a generated test file is through a manual invocation of
clang-tblgen.  A test making use of this emitter will be added in a
followup commit.

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