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

[Rana Pratap Reddy via cfe-commits]

ranapratap55 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.
 

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