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/shadowing/alias/const.wgsl

@@ -0,0 +1,10 @@
+type a = i32;
+
+fn f() {
+  {
+    const a : a = a();
+    const b = a;
+  }
+  const a : a = a();
+  const b = a;
+}

test/tint/shadowing/alias/const.wgsl.expected.glsl

@@ -0,0 +1,11 @@
+#version 310 es
+
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
+void unused_entry_point() {
+  return;
+}
+void f() {
+  {
+  }
+}
+

test/tint/shadowing/alias/const.wgsl.expected.hlsl

@@ -0,0 +1,9 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  {
+  }
+}

test/tint/shadowing/alias/const.wgsl.expected.msl

@@ -0,0 +1,8 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void f() {
+  {
+  }
+}
+

test/tint/shadowing/alias/const.wgsl.expected.spvasm

@@ -0,0 +1,21 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 7
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
+               OpExecutionMode %unused_entry_point LocalSize 1 1 1
+               OpName %unused_entry_point "unused_entry_point"
+               OpName %f "f"
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+%unused_entry_point = OpFunction %void None %1
+          %4 = OpLabel
+               OpReturn
+               OpFunctionEnd
+          %f = OpFunction %void None %1
+          %6 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/tint/shadowing/alias/const.wgsl.expected.wgsl

@@ -0,0 +1,10 @@
+type a = i32;
+
+fn f() {
+  {
+    const a : a = a();
+    const b = a;
+  }
+  const a : a = a();
+  const b = a;
+}