Reapply "[DebugInfo] Alternate (more efficient) MD5 fix"

D155991 changed the file lookup to do a full string compare on the
filename; however, this added ~0.5% to compile time with -g.
Go back to the previous pointer-based lookup, but capture the main
file's checksum as well as its name to use when creating the extra
DIFile entry. This causes all entries to be consistent and also
avoids computing the checksum twice.

This reverts commit 21e7f734.
I'm unable to find a reason for the memory management issues
that caused the revert, so trying again.