Add support for forward declarations of structures.

Now a declaration like 'struct Foo;' can be used to establish the
name of a struct type, without providing a definition.  One can
pass pointers to such types around the system, but can't do much
else with them (as in C/C++).

Issue #125.

tests_errors/deref-3.ispc

@@ -1,4 +1,4 @@
-// Dereference operator "->" can't be applied to non-pointer type "varying struct Foo"
+// Member operator "->" can't be applied to non-pointer type "varying struct Foo"
 
 struct Foo { int x; };