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/const/global/global.wgsl.expected.msl b/test/tint/const/global/global.wgsl.expected.msl
new file mode 100644
index 0000000..1807542
--- /dev/null
+++ b/test/tint/const/global/global.wgsl.expected.msl
@@ -0,0 +1,39 @@
+#include <metal_stdlib>
+
+using namespace metal;
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
+};
+
+struct tint_symbol_1 {
+  float4 value [[color(0)]];
+};
+
+float4 tint_symbol_inner() {
+  int v1 = 1;
+  uint v2 = 1u;
+  float v3 = 1.0f;
+  int3 v4 = int3(1);
+  uint3 v5 = uint3(1u);
+  float3 v6 = float3(1.0f);
+  float3x3 v7 = float3x3(float3(1.0f), float3(1.0f), float3(1.0f));
+  tint_array<float, 10> v9 = tint_array<float, 10>{};
+  return float4(0.0f);
+}
+
+fragment tint_symbol_1 tint_symbol() {
+  float4 const inner_result = tint_symbol_inner();
+  tint_symbol_1 wrapper_result = {};
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}
+