c++: -Waddress and if constexpr [PR94554]

c++: -Waddress and if constexpr [PR94554]

Like we avoid various warnings for seemingly tautological expressions when
substituting a template, we should avoid warning for the implicit conversion
to bool in an if statement.  I considered also doing this for the conditions
in loop expressions, but that seems unnecessary, as a loop condition is
unlikely to be a constant.

The change to finish_if_stmt_cond isn't necessary since dependent_operand_p
looks through IMPLICIT_CONV_EXPR, but makes it more constent with
e.g. build_x_binary_op that determines the type of an expression and then
builds it using the original operands.

	PR c++/94554

gcc/cp/ChangeLog:

	* pt.cc (dependent_operand_p): Split out from...
	(tsubst_copy_and_build): ...here.
	(tsubst_expr) [IF_STMT]: Use it.
	* semantics.cc (finish_if_stmt_cond): Keep the pre-conversion
	condition in the template tree.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-if38.C: New test.