Add a more convenient interface to use clang-format.

It turns out that several implementations go through the trouble of
setting up a SourceManager and Lexer and abstracting this into a
function makes usage easier.

Also abstracts SourceManager-independent ranges out of
tooling::Refactoring and provides a convenience function to create them
from line ranges.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181997 91177308-0d34-0410-b5e6-96231b3b80d8