Ninja: Update showIncludes prefix detection for clang-cl 18

Since commit LLVM/Clang commit 5523fefb01c2 ([clang][lex] Use preferred path separator in includer-relative lookup, 2023-09-08), part of the upcoming 18.x release, the output format of the showIncludes flag has changed, where it now prints paths with double backslashes:

Note: including file: .\\foo.h

Previously, we expected to see the path name in the form "./foo.h".

Extend the regex to match a path name starting with .\, in addition to the existing matched patterns.

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