[clang] [CIR] Enable x86_64 calling-convention lowering by default (PR #215026)

[Andy Kaylor via cfe-commits]

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

I'm OK with this apart from a nit about variable naming, and at least one place where you left a comment in a test saying that there was a difference because of missing ABI support but that was no longer true.

There are a lot of lingering issues with inconsistency with what we're checking, mostly checking more than we really need to for the purpose of a test, but we can either make a pass over the tests later to clean that up or (more likely) clean it up on a case-by-case basis as the tests need to be updated for other reasons.

https://github.com/llvm/llvm-project/pull/215026