Add some entropy to the folder name in Driver/warning-options.cpp.

It was trying to check that things behave correctly when a
non-existant folder was specified for -isysroot.  Incidentally,
I have a folder named FOO in the root of my drive, so this test
was failing.  Make this impossible by using %T to refer to a
definitely non-existant folder.:

llvm-svn: 281998