[llvm] [LLVM][C API] Support Value extraction from a MetadataAsValue. (PR #105501)

[Tim Besard via llvm-commits]

maleadt wrote:

> I'm a bit concerned that this is a silent behavior change in the C API

I guess that's fair. It seemed silly to create a `LLVMMetadataAsValue2` for this, especially given how `LLVMValueAsMetadata` behaves.

Or should we switch to a more explicit API for extracting contained values from a ValueAsMetadata / metadata from a MetadataAsValue? I always found the explicit unwrapping behavior of `LLVMValueAsMetadata` kinda unexpected.

https://github.com/llvm/llvm-project/pull/105501