Fix clang tool test harness to run on Windows.
Fix clang tool test harness to run on Windows.

BUG=495396
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1644313004 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372574}

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