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

[Sven van Haastregt via cfe-commits]

svenvh wrote:

> @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"?

That might not have been the intent, so perhaps this will need some more work adding undef macros similar to those from this PR.

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