[Clangd] Initial version of ExtractFunction

Summary:
- Only works for extraction from free functions
- Basic analysis of the code being extracted.
- Extract to void function
- Bail out if extracting a return, continue or break.
- Doesn't hoist decls yet

Reviewers: kadircet, sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65526

llvm-svn: 370249