Re-land "Implement source directory path processing in unsafe buffers plugin."

Uses llvm::sys::path::Style::posix in several places to avoid path
matching issues on several windows platform.

Bug: 445971865
Change-Id: I5638e2b078a2747e4f18429929e323dcebe7f852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6967286
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1525985}
NOKEYCHECK=True
GitOrigin-RevId: 3fe4c3f0702815a8533243de441d59b734d259ad