Clang frontend changes for OpenCL C Cooperative Matrix Extension

[AaronBallman]

RFC: https://discourse.llvm.org/t/rfc-clang-frontend-changes-for-opencl-c-cooperative-matrix-extension/90148

[AaronBallman]

This was discussed at the Apr 16 Clang Area Team meeting, minutes at: https://docs.google.com/document/d/13-68BCOt8kH5k_56fURBi7u7Ap97Z0EshgkJXDudbJM/edit?usp=sharing

Not yet ready to be accepted, the author is working on updates to the RFC and the implementation.