Revert "Bitcode: Move the DEBUG_LOC record to DEBUG_LOC_OLD"

This reverts commit r225498 (but leaves r225499, which was a worthy
cleanup).

My plan was to change `DEBUG_LOC` to store the `MDNode` directly rather
than its operands (patch was to go out this morning), but on reflection
it's not clear that it's strictly better.  (I had missed that the
current code is unlikely to emit the `MDNode` at all.)

Conflicts:
	lib/Bitcode/Reader/BitcodeReader.cpp (due to r225499)

llvm-svn: 225531