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/statements/for/initializer/array_ctor.wgsl.expected.glsl

@@ -7,7 +7,7 @@ void unused_entry_point() {
 void f() {
   int tint_symbol[1] = int[1](1);
   {
-    for(int i = 1; false; ) {
+    for(int i = tint_symbol[0]; false; ) {
     }
   }
 }

test/tint/statements/for/initializer/array_ctor.wgsl.expected.hlsl

@@ -6,7 +6,7 @@ void unused_entry_point() {
 void f() {
   const int tint_symbol[1] = {1};
   {
-    [loop] for(int i = 1; false; ) {
+    [loop] for(int i = tint_symbol[0]; false; ) {
     }
   }
 }

test/tint/statements/for/initializer/array_ctor.wgsl.expected.msl

@@ -16,7 +16,7 @@ struct tint_array {
 
 void f() {
   tint_array<int, 1> const tint_symbol = tint_array<int, 1>{1};
-  for(int i = 1; false; ) {
+  for(int i = tint_symbol[0]; false; ) {
   }
 }