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

This reverts commit r266507, reapplying r266503 (and r266505
"ValueMapper: Use API from r266503 in unit tests, NFC") completely
unchanged.

I reverted because of a bot failure here:
  http://lab.llvm.org:8011/builders/lld-x86_64-freebsd/builds/16810/

However, looking more closely, the failure was from a host-compiler
crash (clang 3.7.1) when building:
  lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/DwarfAccelTable.cpp.o

I didn't modify that file, or anything it includes, with that commit.

The next build (which hadn't picked up my revert) got past it:
  http://lab.llvm.org:8011/builders/lld-x86_64-freebsd/builds/16811/

I think this was just unfortunate timing.  I suppose the bot must be
flakey.

llvm-svn: 266510