c++: placeholder type constraint inside range-for [PR99869]

In the testcase below, during ahead-of-time deduction of a constrained
auto inside a range-based for loop, we trip over an assert within
do_auto_deduction which expects the deduction context to be
adc_return_type or adc_variable_type, but do_range_for_auto_deduction
calls do_auto_deduction with the context defaulted to adc_unspecified.

We could safely relax the assert to also accept adc_unspecified, but it
seems the deduction context should really be adc_variable_type here.

gcc/cp/ChangeLog:

	PR c++/99869
	* parser.c (do_range_for_auto_deduction): Pass adc_variable_type
	to do_auto_deduction.

gcc/testsuite/ChangeLog:

	PR c++/99869
	* g++.dg/cpp2a/concepts-placeholder6.C: New test.

gcc/cp/parser.c

@@ -12878,7 +12878,9 @@ do_range_for_auto_deduction (tree decl, tree range_expr)
 					    RO_UNARY_STAR,
 					    tf_warning_or_error);
 	  TREE_TYPE (decl) = do_auto_deduction (TREE_TYPE (decl),
-						iter_decl, auto_node);
+						iter_decl, auto_node,
+						tf_warning_or_error,
+						adc_variable_type);
 	}
     }
 }

gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C

@@ -0,0 +1,10 @@
+// PR c++/99869
+// { dg-do compile { target c++20 } }
+
+template <class T, class U> concept same_as = __is_same(T, U);
+
+template <class>
+void f() {
+  for (same_as<int> auto c : "") {} // { dg-error "constraint" }
+  for (same_as<char> auto c : "") {}
+}