[Fix]"[-Wunsafe-buffer-usage] Add a new `forEachDescendant` matcher that skips...

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

The original patch in commit b2ac5fd7
causes compilation errors which can be reproduced by the
`-fdelayed-template-parsing` flag.  This commit fixes the problem.

Related differential revision: https://reviews.llvm.org/D138329