[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/src/CMakeLists.txt b/src/CMakeLists.txt
index 424a4c3..b321263 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -493,6 +493,7 @@
 
 if(${TINT_BUILD_MSL_WRITER})
   list(APPEND TINT_TEST_SRCS
+    writer/msl/generator_impl_return_test.cc
     writer/msl/generator_impl_test.cc
     writer/msl/generator_impl_type_test.cc
   )