BitcodeWriter: Further unify function metadata, NFC

Further unify the handling of function-local metadata with global
metadata, by exposing the same interface in ValueEnumerator.  Both
contexts use the same accessors:

  - getMDStrings(): get the strings for this block.
  - getNonMDStrings(): get the non-strings for this block.

A future commit will start adding strings to the function-block.

llvm-svn: 265224