[clang-tidy][test] Make check_clang_tidy.py work with very long file paths (#155318)
http://github.com/llvm/llvm-project/pull/95220 added a test with a very
long file path, which can fail if run on Windows with a long directory
path.
On Windows, there are file path length limits, which can be worked
around by prefixing the (absolute) path with '\\?\':
https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
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Co-authored-by: Reid Kleckner <rnk@google.com>