[LibTooling] Add "SourceCode" library for functions relating to source-code manipulation.

Summary:
Introduces a utility library in Refactoring/ to collect routines related to
source-code manipulation.  In this change, we move "extended-range" functions
from the FixIt library (in clangTooling) to this new library.

We need to use this functionality in Refactoring/ and cannot access it if it
resides in Tooling/, because that would cause clangToolingRefactor to depend on
clangTooling, which would be a circular dependency.

Reviewers: ilya-biryukov, ioeric

Reviewed By: ilya-biryukov

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60269

llvm-svn: 357764