[llvm-branch-commits] [clang] [clang][CIR] Merge the mlir::cir namespace into cir (PR #115386)

[Shoaib Meenai via llvm-branch-commits]

smeenai wrote:

Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at bottom):
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https://github.com/llvm/llvm-project/pull/115386