[libTooling] Fix bug in Stencil handling of macro ranges

Summary: Currently, an attempt to rewrite source code inside a macro expansion succeeds, but results in empty text, rather than failing with an error.  This patch restructures to the code to explicitly validate ranges before attempting to edit them.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72274