Fix checkBaseABICompatibility() to make it actually check return parameters and not just ordinary parameters twice

test/libsolidity/syntaxTests/abiEncoder/v1_inheritance_from_contract_defining_v2_event.sol

@@ -0,0 +1,15 @@
+==== Source: A ====
+pragma experimental ABIEncoderV2;
+
+struct Item {
+    uint x;
+}
+
+contract C {
+    event Ev(Item);
+}
+==== Source: B ====
+import "A";
+
+contract D is C {}
+// ----

test/libsolidity/syntaxTests/abiEncoder/v1_inheritance_from_contract_defining_v2_function_accepting_struct.sol

@@ -0,0 +1,16 @@
+==== Source: A ====
+pragma experimental ABIEncoderV2;
+
+contract C {
+    struct Item {
+        uint x;
+    }
+
+    function get(Item memory) external view {}
+}
+==== Source: B ====
+import "A";
+
+contract D is C {}
+// ----
+// TypeError 6594: (B:13-31): Contract "D" does not use ABIEncoderV2 but wants to inherit from a contract which uses types that require it. Use "pragma experimental ABIEncoderV2;" for the inheriting contract as well to enable the feature.

test/libsolidity/syntaxTests/abiEncoder/v1_inheritance_from_contract_defining_v2_function_returning_struct.sol

@@ -0,0 +1,16 @@
+==== Source: A ====
+pragma experimental ABIEncoderV2;
+
+contract C {
+    struct Item {
+        uint x;
+    }
+
+    function get() external view returns(Item memory) {}
+}
+==== Source: B ====
+import "A";
+
+contract D is C {}
+// ----
+// TypeError 6594: (B:13-31): Contract "D" does not use ABIEncoderV2 but wants to inherit from a contract which uses types that require it. Use "pragma experimental ABIEncoderV2;" for the inheriting contract as well to enable the feature.