Fix for pr31836 - pp_nonportable_path on absolute paths: broken delimiters

Summary: This is a patch for PR31836. As the bug replaces the path separators in the included file name with the characters following them, the test script makes sure that there's no "Ccase-insensitive-include-pr31836.h" in the warning message.

Reviewers: rsmith, eric_niebler

Reviewed By: eric_niebler

Subscribers: karies, cfe-commits

Differential Revision: https://reviews.llvm.org/D30000

llvm-svn: 295779