c++: partial ordering with memfn ptr cst [PR108104]

c++: partial ordering with memfn ptr cst [PR108104]

Here we're triggering an overzealous assert in unify during partial
ordering since the member function pointer constants are represented as
ordinary CONSTRUCTORs (with TYPE_PTRMEMFUNC_P TREE_TYPE) but the assert
expects COMPOUND_LITERAL_P constructors.

	PR c++/108104

gcc/cp/ChangeLog:

	* pt.cc (unify) <default>: Relax assert to accept any
	CONSTRUCTOR parm, not just COMPOUND_LITERAL_P one.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ptrmem33.C: New test.

(cherry picked from commit 38304846d18d6bb14b0fd6c627c5c6d43a814d01)