[LibTooling] Add retrieval of extended AST-node source to FixIt library

Summary:
Introduces variants of `getText` and `getSourceRange` that extract the source text of an AST node potentially with a trailing token.

Some of the new functions manipulate `CharSourceRange`s, rather than `SourceRange`s, because they document and dynamically enforce their type.  So, this revision also updates the corresponding existing FixIt functions to manipulate `CharSourceRange`s.  This change is not strictly necessary, but seems like the correct choice, to keep the API self-consistent.

This revision is the first in a series intended to improve the abstractions available to users for writing source-to-source transformations.  A full discussion of the end goal can be found on the cfe-dev list with subject "[RFC] Easier source-to-source transformations with clang tooling".

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: kimgr, riccibruno, JonasToth, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58556

llvm-svn: 356095