Reapply "[clang analysis][thread-safety] Handle return-by-reference..… (#68572)

…… (#68394)"

The new warnings are now under a separate flag
`-Wthread-safety-reference-return`, which is on by default under
`-Wthread-safety-reference`.

- People can opt out via `-Wthread-safety-reference
-Wnothread-safety-reference-return`.
This reverts commit 859f2d03.