clang/test/lit.cfg: Improve is_filesystem_case_insensitive() to work properly on cygwin.
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@185323 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang