This PR is part of https://discourse.llvm.org/t/rfc-profile-information-propagation-unittesting/73595
In a slight departure from the RFC, instead of a brand-new `MD_prof_unknown` kind, this adds a first operand to `MD_prof` metadata. This makes it easy to replace with valid metadata (only one `MD_prof`), otherwise sites inserting valid `MD_prof` would also have to check to remove the `unknown` one.
The patch just introduces the notion and fixes the verifier accordingly. Existing APIs working (esp. reading) `MD_prof` will be updated subsequently.
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