clang-modernize: fix invalid assignment in findClangApplyReplacements()

Summary:
When clang-apply-replacements wasn't in the PATH or sitting next to
clang-modernize, findClangApplyReplacements() was finding the wrong path
(pointing to clang-modernize instead of nothing).

See the related PR at http://llvm.org/bugs/show_bug.cgi?id=18421

Reviewers: klimek

CC: silvas, cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3217