c++: bogus warning with value init of const pmf [PR92752]

c++: bogus warning with value init of const pmf [PR92752]

Here we're emitting a -Wignored-qualifiers warning for an intermediate
compiler-generated cast of nullptr to 'method-type* const' as part of
value initialization of a const pmf.  This patch suppresses the warning
by instead casting to the corresponding unqualified type.

	PR c++/92752

gcc/cp/ChangeLog:

	* typeck.cc (build_ptrmemfunc): Cast a nullptr constant to the
	unqualified pointer type not the qualified one.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wignored-qualifiers2.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>