Fix missing returns in libstdc++ header and tests

	* include/bits/locale_conv.h (wbuffer_convert::_M_put): Add missing
	return statement.
	* testsuite/21_strings/basic_string_view/operations/copy/char/1.cc:
	Return void.
	* testsuite/21_strings/basic_string_view/operations/copy/wchar_t/1.cc:
	Likewise.
	* testsuite/23_containers/map/modifiers/insert_or_assign/1.cc: Add
	missing return statements.
	* testsuite/23_containers/unordered_map/modifiers/insert_or_assign.cc:
	Likewise.
	* testsuite/27_io/basic_istream/extractors_arithmetic/char/12.cc:
	Return void.
	* testsuite/special_functions/14_expint/pr68397.cc: Likewise.

From-SVN: r249238