[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
