Rename MDValue* to Metadata* (NFC)

Renamed MDValue* to Metadata*, and MDValueToValIDMap to MetadataToIDs,
as per review for r255909.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256593 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed