[clangd] Allow extract-to-function on regions that always return.

Summary:
We only do a trivial check whether the region always returns - it has to end
with a return statement.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70569