[clang] [CIR] Add metadata type, attributes and metadata_as_value op. (PR #204190)

[via cfe-commits]

skc7 wrote:

> The only coverage is the `-cir-to-llvm` lowering path. A dedicated IR test that verifies !cir.metadata, etc would be good to have.

Thanks. Added a dedicated IR round-trip test.

https://github.com/llvm/llvm-project/pull/204190