c++: Fix consteval operator handling.

We were crashing trying to find the CALL_EXPR in the result of a call to a
consteval operator.

gcc/cp/ChangeLog:

	* call.c (build_new_op_1): Don't look for a CALL_EXPR when
	calling a consteval function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval17.C: New test.

gcc/cp/call.c

@@ -6307,7 +6307,7 @@ build_new_op_1 (const op_location_t &loc, enum tree_code code, int flags,
 	      result = build_over_call (cand, LOOKUP_NORMAL, ocomplain);
 	    }
 
-	  if (trivial_fn_p (cand->fn))
+	  if (trivial_fn_p (cand->fn) || DECL_IMMEDIATE_FUNCTION_P (cand->fn))
 	    /* There won't be a CALL_EXPR.  */;
 	  else if (result && result != error_mark_node)
 	    {

gcc/testsuite/g++.dg/cpp2a/consteval17.C

+// { dg-do compile { target c++20 } }
+
+struct A
+{
+  consteval int operator+() { return 42; }
+};
+
+int main()
+{
+  +A();
+}