[Clangd] ExtractFunction Added checks for broken control flow

Summary:
- Added checks for broken control flow
- Added unittests

Reviewers: sammccall, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66732

llvm-svn: 370455