Implement LWG issue 2306: match_results::reference should be value_type&, not...

Implement LWG issue 2306: match_results::reference should be value_type&, not const value_type&. This is a general move by the LWG to have the reference type of read-only containers be a non-const reference; however, there are no methods that return a non-const reference to a match_result entry, so there's no worries about getting a non-const reference to a constant object.

llvm-svn: 202214