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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202214 91177308-0d34-0410-b5e6-96231b3b80d8