clang/test/lit.cfg: Improve is_filesystem_case_insensitive() to work properly on cygwin.

Cygwin does not accept the form /CYGDRIVE/X/PATH/TO/FILE against /cygdrive/X/PATH/TO/FILE.
"cygdrive" must be lower-cased.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185323 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed