[clang-tidy] Prepare modernize-loop-convert for upcoming changes in D22566

Summary:
D22566 will change RecursiveASTVisitor so that it descends into the initialization expressions for lambda captures.

modernize-loop-convert needs to be prepared for this so that it does not interpret these initialization expressions as invalid uses of the loop variable. The change has no ill effects without D22566 in place, i.e. the change does not depend on D22566.

Reviewers: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22903

llvm-svn: 277339