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

Note this is a change local to -Wunsafe-buffer-usage checks.

Add a new matcher `forEveryDescendant` that recursively matches
descendants of a `Stmt` but skips nested callable definitions.  This
matcher has same effect as using `forEachDescendant` and skipping
`forCallable` explicitly but does not require the AST construction to be
complete.

Reviewed by: NoQ, xazax.hun

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