[clang] [CIR] Add Direct coerce-in-registers + cir.reinterpret_cast op (PR #195879)

[via cfe-commits]

adams381 wrote:

Rebased onto the [#195745](https://github.com/llvm/llvm-project/pull/195745) tip now that [#195737](https://github.com/llvm/llvm-project/pull/195737) has merged. PR is now two commits (Extend + this); CI failure was a stale `rewriter` reference fixed in the rebase.

https://github.com/llvm/llvm-project/pull/195879