[metal-writer] Emit assignment statements.

This CL adds assignment statements to the Metal backend.

Bug: tint:8
Change-Id: Iaf4faa62124948fd0e785d5bebd20aae778ba050
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23705
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index ba497fc..82a51a4 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_assign_test.cc
     writer/msl/generator_impl_function_test.cc
     writer/msl/generator_impl_identifier_test.cc
     writer/msl/generator_impl_return_test.cc