BitcodeWriter: Simplify tracking of function-local metadata, NFC

We don't really need a separate vector here; instead, point at a range
inside the main MDs array.  This matches how r264551 references the
ranges of strings and non-strings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264552 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed