[clang] [CIR] Add Extend (signext/zeroext) handling to CallConvLowering (PR #195745)

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adams381 wrote:

Rebased onto main now that [#195737](https://github.com/llvm/llvm-project/pull/195737) has merged. PR is now a single commit; diff is the Extend handler only.

https://github.com/llvm/llvm-project/pull/195745