Revert "Revert "[-Wunsafe-buffer-usage] Add a new `forEachDescendant` matcher...

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

This reverts commit f58b0253.

 The previous revert reverts a patch that causes compilation problem on
 windows which can be reproduced using `-fdelayed-template-parsing`.
 I'm now to revert the patch back and commit a fix next.