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/condition/array_ctor.wgsl.expected.glsl

@@ -8,7 +8,7 @@ void f() {
   int i = 0;
   while (true) {
     int tint_symbol[1] = int[1](1);
-    if (!((i < 1))) {
+    if (!((i < tint_symbol[0]))) {
       break;
     }
     {

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

@@ -7,7 +7,7 @@ void f() {
   int i = 0;
   [loop] while (true) {
     const int tint_symbol[1] = {1};
-    if (!((i < 1))) {
+    if (!((i < tint_symbol[0]))) {
       break;
     }
     {

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

@@ -18,7 +18,7 @@ void f() {
   int i = 0;
   while (true) {
     tint_array<int, 1> const tint_symbol = tint_array<int, 1>{1};
-    if (!((i < 1))) {
+    if (!((i < tint_symbol[0]))) {
       break;
     }
     {