tint: Deprecated module-scope 'let' for 'const'

Enable the parsing of 'const'.
Warn on use of module-scope 'let', and automatically replace with 'const'.

Fixed: tint:1580
Change-Id: I214aabca80686dc6b60ae21a7a57fbfb4898ea83
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93786
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/expressions/index/let/literal/array.wgsl.expected.msl

@@ -16,6 +16,6 @@ struct tint_array {
 
 int f() {
   tint_array<int, 8> const a = tint_array<int, 8>{1, 2, 3, 4, 5, 6, 7, 8};
-  return 2;
+  return a[1];
 }