[llvm-pdbutil] Allow diff to force module equivalencies.

Sometimes the normal module equivalence detection algorithm doesn't
quite work.  For example, you might build the same program with
MSVC and clang-cl, outputting to different object files, exes, and
PDBs, then compare them.  If the object files have different names
though, then they won't be treated as equivalent.  This way we
can force specific module indices to be treated as equivalent.

llvm-svn: 309983