Strip /showIncludes from the CompDb

/showIncludes causes clang-tidy to output a lot of unnecessary text to
the console.

BUG=944686

Change-Id: I9e8b2e0912d47c70aef9fae8919ab492a310fb29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538645
Auto-Submit: Robert Liao <robliao@chromium.org>
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644457}