Revert "ValueMapper: Eliminate cross-file co-recursion, NFC"

This reverts commit r266503, in case it's the root cause of this bot
failure:

  http://lab.llvm.org:8011/builders/lld-x86_64-freebsd/builds/16810

I'm also reverting r266505 -- "ValueMapper: Use API from r266503 in unit
tests, NFC" -- since it's in the way.

llvm-svn: 266507