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

[Adam Smith via cfe-commits]

adams381 wrote:

@erichkeane You are fast!  I was mid update on the PR body when you approved.  The flag rename is recorded there.

Waiting for @andykaylor to have a chance to look at this.

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