libstdc++: Initialize member in std::match_results [PR103549]

libstdc++: Initialize member in std::match_results [PR103549]

This fixes a -Wuninitialized warning for std::cmatch m1, m2; m1=m2;

Also name the template parameters in the forward declaration, to get rid
of the <template-parameter-1-1> noise in diagnostics.

libstdc++-v3/ChangeLog:

	PR libstdc++/103549
	* include/bits/regex.h (match_results): Give names to template
	parameters in first declaration.
	(match_results::_M_begin): Add default member-initializer.