[metal-writer] Add return generation.

This CL adds generation of the return statement.

Bug: tint:8
Change-Id: Iffee600e77a485649b987d39aab47742968e438e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23702
Reviewed-by: David Neto <dneto@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index e943951..ad647f4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -885,6 +885,7 @@
 
 source_set("tint_unittests_msl_writer_src") {
   sources = [
+    "src/writer/msl/generator_impl_return_test.cc",
     "src/writer/msl/generator_impl_test.cc",
     "src/writer/msl/generator_impl_type_test.cc",
   ]