[LibTooling] Add insert/remove convenience functions for creating `ASTEdit`s.

Summary: `change()` is an all purpose function; the revision adds simple shortcuts for the specific operations of inserting (before/after) or removing source.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62621

llvm-svn: 362707