libstdc++: Add allocator-extended constructors to std::match_results (LWG 2195)

This was approved in Issaquah last month.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (match_results): Add allocator-extended
	copy and move constructors, as per LWG 2195.
	* testsuite/28_regex/match_results/ctors/char/alloc.cc: New test.