The use of #line in the testcases in the appended patch is
questionable, but as a data point the testcases fail without the patch
and pass with the patch.  These versions of the testcases have been
amended to use substitute-path.

ref: pr 2360
http://sourceware.org/ml/gdb/2007-11/msg00138.html
http://sourceware.org/ml/gdb-patches/2007-11/msg00314.html

Maybe #line + substitute-path fixup isn't supported.
Again, this patch is just a data point.

Note: This patch contains the rewrite_subfile_path suggestion of my
previous email.  I don't know whether it helps or hurts the case
you're trying to fix.