c++: Abbreviated function template return type [PR92187]

When updating an auto return type of an abbreviated function template in
splice_late_return_type, we should also propagate PLACEHOLDER_TYPE_CONSTRAINTS
(and cv-qualifiers) of the original auto node.

gcc/cp/ChangeLog:

	PR c++/92187
	* pt.c (splice_late_return_type): Propagate cv-qualifiers and
	PLACEHOLDER_TYPE_CONSTRAINTS from the original auto node to the new one.

gcc/testsuite/ChangeLog:

	PR c++/92187
	* g++.dg/concepts/abbrev5.C: New test.
	* g++.dg/concepts/abbrev6.C: New test.