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

core/odin/ast/ast.odin

@@ -700,6 +700,7 @@ Proc_Type :: struct {
 
 Pointer_Type :: struct {
 	using node: Expr,
+	tag:     ^Expr,
 	pointer: tokenizer.Pos,
 	elem:    ^Expr,
 }