[clang-query] Add comment token handling

Summary:
It is possible to pass a file of commands to clang-query using the
command line option -f or --preload.  Make it possible to write comments
in such files.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D52752

llvm-svn: 343666