getting the on-disk case for a path

[Jeremy Drake]

On a case-insensitive but case-preserving filesystem, is there a Cygwin
API to get the on-disk case for a given path?  It seems like `realpath`
ought to do it but running
$ touch case-test
$ realpath CASE-TEST
returns CASE-TEST.  Regardless, canonicalize_file_name or realpath may not
be what I want because it would dereference symlinks.

Background: I'm trying to debug some test failures in Clang, due to a
warning that's supposed to be issued when you #include "foo.h" but the
file on disk that it opened is "Foo.h".