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>
diff --git a/test/tint/benchmark/shadow-fragment.wgsl b/test/tint/benchmark/shadow-fragment.wgsl
index 29eee86..c2da838 100644
--- a/test/tint/benchmark/shadow-fragment.wgsl
+++ b/test/tint/benchmark/shadow-fragment.wgsl
@@ -21,9 +21,9 @@
   fragNorm : vec3<f32>,
 }
 
-let albedo : vec3<f32> = vec3<f32>(0.899999976, 0.899999976, 0.899999976);
+const albedo : vec3<f32> = vec3<f32>(0.899999976, 0.899999976, 0.899999976);
 
-let ambientFactor : f32 = 0.200000003;
+const ambientFactor : f32 = 0.200000003;
 
 @fragment
 fn main(input : FragmentInput) -> @location(0) vec4<f32> {