Add #soa pointer type to aid with refactoring to #soa data types

a: #soa[16]Foo
p := &a[6]
#assert(type_of(p) == #soa^#soa[16]Foo)
p^.x = 123
p.x = 123

src/parser.hpp

@@ -650,7 +650,8 @@ AST_KIND(_TypeBegin, "", bool) \
 	}) \
 	AST_KIND(PointerType, "pointer type", struct { \
 		Token token; \
-		Ast *type; \
+		Ast *type;   \
+		Ast *tag;    \
 	}) \
 	AST_KIND(RelativeType, "relative type", struct { \
 		Ast *tag; \