[clang] [clang][AVR] Add basic AVR builtin functions (PR #203214)

[Ben Shi via cfe-commits]

https://github.com/benshi001 approved this pull request.

LGTM. The build failures are due to dependancy on https://github.com/llvm/llvm-project/pull/203212. 

Please also let @Patryk27 have a look on this frontend PR.

https://github.com/llvm/llvm-project/pull/203214