xfail case sensitivity test on Linux.

There are two tests in this file.  One which only runs on Windows
and tests that you can set a breakpoint with mismatched case.  And
another that only runs on non-Windows and tests that you cannot set
a breakpoint with mismatched case.  This latter test is failing on
non Windows platforms for some reason.  It could be that the test
is just written incorrectly, as I think the actual functionality
actually works correctly on non-Windows platforms.

llvm-svn: 261800