[clang-tidy] Make NumOccurrenceFlag for SourcePaths configurable.

Added an additional ctor that takes a NumOccurrenceFlag parameter for the
SourcePaths option. This frees applications from always having to pass at least
one source file, e.g., -list-checks.

http://reviews.llvm.org/D12069

Patch by Don Hinton!

llvm-svn: 245204