[refactor][extract] insert semicolons into extracted/inserted code

when needed

This commit implements the semicolon insertion logic into the extract
refactoring. The following rules are used:

- extracting expression: add terminating ';' to the extracted function.
- extracting statements that don't require terminating ';' (e.g. switch): add
  terminating ';' to the callee.
- extracting statements with ';':  move (if possible) the original ';' from the
  callee and add terminating ';'.
- otherwise, add ';' to both places.

Differential Revision: https://reviews.llvm.org/D39441

llvm-svn: 317343