c++: Fix wrong no post-decrement operator error in template [PR94190]

c++: Fix wrong no post-decrement operator error in template [PR94190]

Now that convert_like creates an IMPLICIT_CONV_EXPR when it converts
something that involves a class in a template, we must be prepared to
handle it.  In this test, we have a class S and we're converting it
to long int& using a user-defined conversion since we're performing
-- on it.  So cp_build_unary_op/POSTDECREMENT_EXPR calls
build_expr_type_conversion which gets the IMPLICIT_CONV_EXPR.  Before
the convert_like change it got *S::operator long int &(&b) whose type
is long int but now it gets IMPLICIT_CONV_EXPR<long int&>(b) whose type
is a reference type.  But the !MAYBE_CLASS_TYPE_P switch doesn't handle
reference types and so we complain.

Fixed by calling convert_from_reference on the result of convert_like.

	PR c++/94190 - wrong no post-decrement operator error in template.
	* call.c (convert_like_real): Use convert_from_reference on the result.

	* g++.dg/conversion/op7.C: New test.