[msl-writer] Emit alias types.

This CL adds emission of alias types to the MSL writer. They are output
as `typedef`s.

Bug: tint:8
Change-Id: I18b839ed0238c4636e3975f35f5d88badd412fe4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/24001
Reviewed-by: David Neto <dneto@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index b9b93bf..0187d06 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -884,6 +884,7 @@
 
 source_set("tint_unittests_msl_writer_src") {
   sources = [
+    "src/writer/msl/generator_impl_alias_type_test.cc",
     "src/writer/msl/generator_impl_array_accessor_test.cc",
     "src/writer/msl/generator_impl_as_test.cc",
     "src/writer/msl/generator_impl_assign_test.cc",