Resubmit "Add pdb-diff test."

This was originally reverted because of two issues.
  1) Printing ANSI color escape codes even when outputting to
     a file
  2) Module name comparisons were failing when comparing a PDB
     generated on one machine to a PDB generated on another
     machine.

I attempted to fix #2 by adding command line options which let
you specify prefixes to strip from the beginning of embedded
paths, which effectively lets us specify a path to "base" each
PDB from and only compare the parts under the base.  But this is
tricky because PDB paths always use Windows path syntax, even
when they are created on non-Windows hosts.  A problem still
existed when constructing the prefix to strip, where we were
accidentally using a host-specific path separator instead of
a Windows path separator.

This resubmission fixes the issue on Linux (and I have verified
that the test now passes on Linux).

llvm-svn: 307571