make PointerKind directly reflect pointer types

The code that consumes PointerKind (`adjust_for_rust_scalar` in rustc_ty_utils)
ended up using PointerKind variants to talk about Rust reference types (& and
&mut) anyway, making the old code structure quite confusing: one always had to
keep in mind which PointerKind corresponds to which type. So this changes
PointerKind to directly reflect the type.

This does not change behavior.

tests/codegen/function-arguments-noopt.rs

@@ -29,6 +29,12 @@ pub fn borrow(x: &i32) -> &i32 {
   x
 }
 
+// CHECK: align 4 {{i32\*|ptr}} @borrow_mut({{i32\*|ptr}} align 4 %x)
+#[no_mangle]
+pub fn borrow_mut(x: &mut i32) -> &mut i32 {
+  x
+}
+
 // CHECK-LABEL: @borrow_call
 #[no_mangle]
 pub fn borrow_call(x: &i32, f: fn(&i32) -> &i32) -> &i32 {