Rename MapValue(Metadata*) to MapMetadata()

Instead of reusing the name `MapValue()` when mapping `Metadata`, use
`MapMetadata()`.  The old name doesn't make much sense after the
`Metadata`/`Value` split.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224566 91177308-0d34-0410-b5e6-96231b3b80d8