[clang-tidy] Allow use of -list-checks option without need to pass source files.

Initialize CommonOptionsParser with ZeroOrOne NumOccurrenceFlag so callers can
pass -list-checks without the need to pass additional positional parameters,
then add dummy file if none were supplied.

http://reviews.llvm.org/D12070

Patch by Don Hinton!

llvm-svn: 245205