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/vector.wgsl.expected.glsl

@@ -6,6 +6,6 @@ void unused_entry_point() {
 }
 float f() {
   vec3 v = vec3(1.0f, 2.0f, 3.0f);
-  return 2.0f;
+  return v[1];
 }