tint/writer/wgsl: Print @offset attributes as comment

We use WGSL to visualize the AST. Make sure we don't hide anything.

Bug: tint:1776
Change-Id: Iedd7ca797fb745d9db7d0aba8a5718039241afbb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/112602
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>

test/tint/builtins/atomicStore/struct/array_of_struct.spvasm.expected.wgsl

@@ -1,12 +1,18 @@
 struct S_atomic {
+  /* @offset(0) */
   x : i32,
+  /* @offset(4) */
   a : atomic<u32>,
+  /* @offset(8) */
   y : u32,
 }
 
 struct S {
+  /* @offset(0) */
   x : i32,
+  /* @offset(4) */
   a : u32,
+  /* @offset(8) */
   y : u32,
 }