Ninja: Restore detection of clang-cl showIncludes prefix

Since !8129 (merged) our regex no longer matches the output from clang-cl, which uses a relative path, forward slashes, and is always in English:

Note: including file: ./foo.h

Update the regex to match that too.

Original Description

MSVC: Fix /showIncludes prefix compatibility for clang-cl

clang-cl, while having MSVC-like command-line, doesn't always have identical output with cl, eg. with include notes.

Include notes emitted by clang-cl use / as path separator and may use relative paths, so the compatibility was broken by !8129 (merged).

This MR tries to fix the compatibility with clang-cl, and add a test case to ensure we won't break it accidentally.

Backport: release
Topic-rename: clang-cl-showIncludes