Do not display highlights for clang-include-fixer-at-point

Summary: When invoking clang-include-fixer-at-point, the QuerySymbolInfos point to offset 0, length 0. Rather than showing a hidden overlay, do not show any overlay at all for zero-length symbols.

Patch by Torsten Marek!

Reviewers: hokein, klimek

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D30577

llvm-svn: 297010