Resubmit "Delete the remainder of platform specific code in FileSpec."

This was causing a test failure in one of LLDB's tests which
specifically dealt with a limitation in LLVM's implementation
of home_directory() that LLDB's own implementation had worked
around.

This limitation has been addressed in r298513 on the LLVM side,
so the failing test (which is now unnecessary as the limitation
no longer exists) was removed in r298519, allowing this patch to
be re-submitted without modification.

llvm-svn: 298526