[OpenCL] Add builtin declarations by default.

This change enables the builtin function declarations
in clang driver by default using the Tablegen solution
along with the implicit include of 'opencl-c-base.h'
header.

A new flag '-cl-no-stdinc' disabling all default
declarations and header includes is added. If any other
mechanisms were used to include the declarations (e.g.
with -Xclang -finclude-default-header) and the new default
approach is not sufficient the, `-cl-no-stdinc` flag has
to be used with clang to activate the old behavior.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D96515