Make -Wnonportable-include-path ignore drive case on Windows.

See PR45812 for motivation.

No explicit test since I couldn't figure out how to get the
current disk drive in lower case into a form in lit where I could
mkdir it and cd to it. But the change does have test coverage in
that I can remove the case normalization in lit, and tests failed
on several bots (and for me locally if in a pwd with a lower-case
drive) without that normalization prior to this change.

Differential Revision: https://reviews.llvm.org/D79531