c++: function NTTP argument considered unused [PR53164, PR105848]

c++: function NTTP argument considered unused [PR53164, PR105848]

Here at parse time the template argument f (an OVERLOAD) in A<f> gets
resolved ahead of time to the FUNCTION_DECL f<int>, and we defer marking
f<int> as used until instantiation (of g) as usual.

Later when instantiating g the type A<f> (where f has already been
resolved) is non-dependent, so tsubst_aggr_type avoids re-processing its
template arguments, and we end up never actually marking f<int> as used
(which means we never instantiate it) even though A<f>::h() later calls
it, leading to a link error.

This patch works around this issue by looking through ADDR_EXPR when
calling mark_used on the substituted callee of a CALL_EXPR.

	PR c++/53164
	PR c++/105848

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Look through an
	ADDR_EXPR callee when calling mark_used.

gcc/testsuite/ChangeLog:

	* g++.dg/template/fn-ptr3.C: New test.

(cherry picked from commit 733a792a2b2e1662e738fa358b45a2720a8618a7)