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/const/inferred/function.wgsl

@@ -0,0 +1,21 @@
+type MyArray = array<f32, 10>;
+
+// Function-scope consts
+fn const_decls() {
+    const v1 = 1;
+    const v2 = 1u;
+    const v3 = 1.0;
+
+    const v4 = vec3<i32>(1, 1, 1);
+    const v5 = vec3<u32>(1u, 1u, 1u);
+    const v6 = vec3<f32>(1.0, 1.0, 1.0);
+
+    const v7 = mat3x3<f32>(v6, v6, v6);
+
+    const v8 = MyArray();
+}
+
+@fragment
+fn main() -> @location(0) vec4<f32> {
+    return vec4<f32>(0.0,0.0,0.0,0.0);
+}