[clang] [Clang][Driver] Add option to provide path for multilib's YAML config file (PR #109640)

[via cfe-commits]

https://github.com/ArcaneNibble requested changes to this pull request.

You need to check the command line arg inside `BareMetal::findMultilibs`, as that function does not call `findMultilibsFromYAML` at all if the default path does not exist. (This was picked up in CI.)

https://github.com/llvm/llvm-project/pull/109640