cg_llvm: Replace enum MetadataType with a list of MetadataKindId constants

[Zalathar]

The metadata kind ID values declared in MetadataType are not part of the LLVM-C API, and are not machine-checked. If a value that we use ever goes out of sync with LLVM, the resulting bugs could be difficult to track down. And the existing values lack any clear indication of what LLVM declarations they correspond to.

On top of that, we currently have another way of expressing metadata kind IDs in the form of MetadataKindId, which creates confusing inconsistency in LLVM bindings.

This PR therefore consolidates all usage of “fixed” metadata kind IDs into one list of MetadataKindId constants, which is backed by static assertions in our C++ code that match them up with named anonymous-enum variants in llvm::LLVMContext.

[rustbot]

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