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; };
 

tests_errors/struct-ref-undecl-1.ispc

@@ -0,0 +1,5 @@
+// Member operator "." can't be applied to declared but not defined struct type
+
+struct Foo;
+
+int bar(Foo & foo) { return foo.x; }

tests_errors/struct-ref-undecl-2.ispc

@@ -0,0 +1,5 @@
+// Member operator "->" can't be applied to declared but not defined struct type
+
+struct Foo;
+
+int bar(Foo * uniform foo) { return foo->x; }