[clang] [Clang][OpenCL] Promote a few extensions to OpenCL 3.1 core (PR #204330)

[Wenju He via cfe-commits]

wenju-he wrote:

@svenvh after confirmation with Ben Ashbaugh, the latest commit changes OPENCL_OPTIONALCOREFEATURE to OPENCL_COREFEATURE because `core` means feature is mandatory and must always be supported by any 3.1 implementation.
please review again, thanks.

> Are you aware of any other missing OpenCL C 3.1 functionality? If not, we should probably try to backport this change to the 23.x release branch too.

Confirmed with with Ben Ashbaugh, no missing OpenCL C 3.1 functionality. This PR together with https://github.com/llvm/llvm-project/pull/204043 and https://github.com/llvm/llvm-project/pull/208370 are complete.
Will backport this PR to 23.x, thanks for the suggestion.

https://github.com/llvm/llvm-project/pull/204330