[clang] [CIR] Change default assumption about allowing builtins (PR #144004)

[Erich Keane via cfe-commits]

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

Seems like a reasonable assumption for now.  THOUGH, I wonder if we'd be better off at one point instead to figure out which ones we DO know how to handle, and use the 'nobuiltin' as a fallback instead?  That way we can exercise the builtin-code as it gets implemented.

https://github.com/llvm/llvm-project/pull/144004