[LibTooling] Add "SourceCode" library for functions relating to source-code manipulation.
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
Differential Revision: https://reviews.llvm.org/D60269