[OpenCL] Allow undefining header-only features

[OpenCL] Allow undefining header-only features

`opencl-c-base.h` always defines 5 particular feature macros for
SPIR-V, making it impossible to disable those features.

To allow disabling any of those features, let the header recognize
`__undef_<feature>` macros.  The user can then pass the
`-D__undef_<feature>` flag on the command line to disable a specific
feature.  The __undef macro could potentially also be set from
`-cl-ext=-feature`, but for now only change the header and only
provide __undef macros for the 5 features that are always enabled in
`opencl-c-base.h`.

Differential Revision: https://reviews.llvm.org/D141297