Some fixes for case insensitive paths on Windows.

Paths on Windows are not case-sensitive.  Because of this, if a file
is called main.cpp, you should be able to set a breakpoint on it
by using the name Main.cpp.  In an ideal world, you could just
tell people to match the case, but in practice this can be a real
problem as it requires you to know whether the person who compiled
the program ran "clang++ main.cpp" or "clang++ Main.cpp", both of
which would work, regardless of what the file was actually called.

This fixes http://llvm.org/pr22667

Patch by Petr Hons

Differential Revision: http://reviews.llvm.org/D17492
Reviewed by: zturner

llvm-svn: 261771