Fix type check for nested arrays in abi.encode/decode functions in ABIEncoderV1

- Without this fix, nested arrays are not detected as unsupported and compiler fails on an UnimplementedError.
- Now it's consistent with how structs are handled in ABIEncoderV1.

test/libsolidity/syntaxTests/specialFunctions/abidecode/abi_decode_struct.sol

@@ -0,0 +1,11 @@
+struct S {
+    uint x;
+}
+
+contract C {
+    function f() public pure {
+        abi.decode("1234", (S));
+    }
+}
+// ----
+// TypeError 9611: (98-99): Decoding type struct S memory not supported.