[clang] [clang] Implement C2y stdc_load8_* endian-aware 8-bit load functions (PR #203666)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM but please give @efriedma-quic a day or two before landing in case he has additional review feedback, particularly on the codegen bits.

https://github.com/llvm/llvm-project/pull/203666