Add optional access to ptr<>

This also completes the work to resolve the access controls for each
storage type.

Fixed: tint:846
Change-Id: Iab24057ec14620a2978ec63c4a91ba12d1bc6e9b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53381
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

test/ptr_ref/load/local/ptr_storage.wgsl

@@ -8,6 +8,6 @@ var<storage, read_write> v : S;
 
 [[stage(compute)]]
 fn main() {
-  let p : ptr<storage, i32> = &v.a;
+  let p : ptr<storage, i32, read_write> = &v.a;
   let use : i32 = *p + 1;
 }

test/ptr_ref/load/local/ptr_uniform.wgsl.expected.spvasm

@@ -13,6 +13,7 @@
                OpName %main "main"
                OpDecorate %S Block
                OpMemberDecorate %S 0 Offset 0
+               OpDecorate %v NonWritable
                OpDecorate %v DescriptorSet 0
                OpDecorate %v Binding 0
         %int = OpTypeInt 32 1