Issue errors when doing illegal things with incomplete struct types.

Issue an error, rather than crashing, if the user has declared a
struct type but not defined it and subsequently tries to:

- dynamically allocate an instance of the struct type
- do pointer math with a pointer to the struct type
- compute the size of the struct type

tests_errors/undef-struct-ptrmath.ispc

@@ -0,0 +1,7 @@
+// Illegal to pre/post increment pointer to undefined struct type
+
+struct Foo;
+
+Foo * uniform bar(Foo * uniform f) {
+    return ++f;
+}