[clang] [OpenCL] Allow undefining cl_khr_integer_dot_product features macros (PR #162055)

[Alexey Bader via cfe-commits]

bader wrote:

> > For simplicity, can we revert https://reviews.llvm.org/D92231 and don't always define the feature macro in headers?
> 
> Not really, reverting that does not solve the underlying issue and would be a step backwards regarding moving header-only extensions out of the Clang internals. If I understand correctly, the underlying issue is that historically the `SPIR` target had become the equivalent of "enable all known extensions". Perhaps that's something we'd want to reconsider, but that's a separate discussion.

My understanding is `SPIR` target enables all known extensions by default, but I expect that defaults can be overridden. 
@svenvh, do I get it right that moving "header-only extensions out of the Clang internals" removes the option to configure the defaults for "header-only extensions"?

https://github.com/llvm/llvm-project/pull/162055