[OpenCL] Declare builtin functions using TableGen

This patch adds a `-fdeclare-opencl-builtins` command line option to
the clang frontend.  This enables clang to verify OpenCL C builtin
function declarations using a fast StringMatcher lookup, instead of
including the opencl-c.h file with the `-finclude-default-header`
option.  This avoids the large parse time penalty of the header file.

This commit only adds the basic infrastructure and some of the OpenCL
builtins.  It does not cover all builtins defined by the various OpenCL
specifications.  As such, it is not a replacement for
`-finclude-default-header` yet.

RFC: http://lists.llvm.org/pipermail/cfe-dev/2018-November/060041.html

Co-authored-by: Pierre Gondois
Co-authored-by: Joey Gouly
Co-authored-by: Sven van Haastregt

Differential Revision: https://reviews.llvm.org/D60763

llvm-svn: 362371