Re-land "[-Wunsafe-buffer-usage] Add a new `forEachDescendant` matcher that...

Re-land "[-Wunsafe-buffer-usage] Add a new `forEachDescendant` matcher that skips callable declarations"

This reverts commit 22df4549.

After a quick investigation, realizing that the Sanitizer test
failures caused by this patch is not likely to block other
contributors. I re-land this patch before taking a closer look at
those tests so that it won't block the [-Wunsafe-buffer-usage]
development.