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

[Wenju He via cfe-commits]

wenju-he wrote:

For simplicity, can we revert https://reviews.llvm.org/D92231 and don't always define the feature macro in headers?
It would be easier for users. For instance, as @aratajew points out, Mesa workarounds the issue by undefining "__SPIR__" and `"__SPIRV__"`, see https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/compiler/clc/clc_helpers.cpp#L946

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