[OpenCL] Split type and macro definitions into opencl-c-base.h

[OpenCL] Split type and macro definitions into opencl-c-base.h

Using the -fdeclare-opencl-builtins option will require a way to
predefine types and macros such as `int4`, `CLK_GLOBAL_MEM_FENCE`,
etc.  Move these out of opencl-c.h into opencl-c-base.h such that the
latter can be shared by -fdeclare-opencl-builtins and
-finclude-default-header.

This changes the behaviour of -finclude-default-header when
-fdeclare-opencl-builtins is specified: instead of including the full
header, it will include the header with only the base definitions.

Differential revision: https://reviews.llvm.org/D63256

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363794 91177308-0d34-0410-b5e6-96231b3b80d8