[clang-tidy] Fix llvm-header-guard so that it works with Windows paths

Fixes pr40372 (https://bugs.llvm.org/show_bug.cgi?id=40372).

The llvm-header-guard check does not take into account that the path
separator on Windows is `\`, not `/`.

This means that instead of suggesting a header guard in the form of:
LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_FOO_H

it incorrectly suggests:
C:\LLVM_PROJECT\CLANG_TOOLS_EXTRA\CLANG_TIDY\FOO_H

Differential Revision: https://reviews.llvm.org/D113450