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/let/inferred/function.wgsl.expected.hlsl b/test/tint/let/inferred/function.wgsl.expected.hlsl
index 5a75ea6..152e8d1 100644
--- a/test/tint/let/inferred/function.wgsl.expected.hlsl
+++ b/test/tint/let/inferred/function.wgsl.expected.hlsl
@@ -32,7 +32,7 @@
   const int3 v4 = (1).xxx;
   const uint3 v5 = (1u).xxx;
   const float3 v6 = (1.0f).xxx;
-  const float3x3 v7 = float3x3((1.0f).xxx, (1.0f).xxx, (1.0f).xxx);
+  const float3x3 v7 = float3x3(v6, v6, v6);
   const MyStruct v8 = {1.0f};
   const float v9[10] = (float[10])0;
   const int v10 = ret_i32();
diff --git a/test/tint/let/inferred/function.wgsl.expected.msl b/test/tint/let/inferred/function.wgsl.expected.msl
index 604c979..440faea 100644
--- a/test/tint/let/inferred/function.wgsl.expected.msl
+++ b/test/tint/let/inferred/function.wgsl.expected.msl
@@ -47,7 +47,7 @@
   int3 const v4 = int3(1);
   uint3 const v5 = uint3(1u);
   float3 const v6 = float3(1.0f);
-  float3x3 const v7 = float3x3(float3(1.0f), float3(1.0f), float3(1.0f));
+  float3x3 const v7 = float3x3(v6, v6, v6);
   MyStruct const v8 = {.f1=1.0f};
   tint_array<float, 10> const v9 = tint_array<float, 10>{};
   int const v10 = ret_i32();
diff --git a/test/tint/let/inferred/function.wgsl.expected.spvasm b/test/tint/let/inferred/function.wgsl.expected.spvasm
index e3f36f6..6e2ddcb 100644
--- a/test/tint/let/inferred/function.wgsl.expected.spvasm
+++ b/test/tint/let/inferred/function.wgsl.expected.spvasm
@@ -50,7 +50,6 @@
     %v3float = OpTypeVector %float 3
          %40 = OpConstantComposite %v3float %float_1 %float_1 %float_1
 %mat3v3float = OpTypeMatrix %v3float 3
-         %42 = OpConstantComposite %mat3v3float %40 %40 %40
          %43 = OpConstantComposite %MyStruct %float_1
          %50 = OpTypeFunction %v4float
     %ret_i32 = OpFunction %int None %6
@@ -75,6 +74,7 @@
                OpFunctionEnd
   %let_decls = OpFunction %void None %31
          %34 = OpLabel
+         %42 = OpCompositeConstruct %mat3v3float %40 %40 %40
          %44 = OpFunctionCall %int %ret_i32
          %45 = OpFunctionCall %uint %ret_u32
          %46 = OpFunctionCall %float %ret_f32