This groups all the constructors together, consistent with the synopses
in the C++20 standard.

libstdc++-v3/ChangeLog:

	* include/std/sstream (basic_stringbug, basic_istringstream)
	(basic_ostringstream, basic_stringstream): Reorder C++20
	constructors to be declared next to other constructors.

Tested powerpc64le-linux. Committed to trunk.