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

The original patch does include a `new` statement without a matching
`delete`, causing Sanitizer warnings in
https://lab.llvm.org/buildbot/#/builders/5/builds/30522/steps/13/logs/stdio.

This commit is a fix to it.

Differential Revision: https://reviews.llvm.org/D138329