[Clang][OpenCL] Add OpenCL 3.1 language version (#204043)
OpenCL 3.1 was recently added to spec in
https://github.com/KhronosGroup/OpenCL-Docs/commit/9fff1a87a975
This PR adds version 3.1 to clang support, including frontend flag
`-std=cl3.1` (and its alias `-std=CL3.1`).
Updated release note.
A few extensions are promoted to 3.1 core. They'll be handled in
follow-up PRs.
Assisted-by: Claude Sonnet 4.6
