Implement phony assignment

Bug: tint:1213
Change-Id: Ib1ebc4947405c4ada7a9bdbc6bd5a36447bbd234
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67064
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/test/statements/assign/phony/uniform_buffer.wgsl.expected.msl b/test/statements/assign/phony/uniform_buffer.wgsl.expected.msl
new file mode 100644
index 0000000..6da18d2
--- /dev/null
+++ b/test/statements/assign/phony/uniform_buffer.wgsl.expected.msl
@@ -0,0 +1,11 @@
+#include <metal_stdlib>
+
+using namespace metal;
+struct S {
+  /* 0x0000 */ int i;
+};
+
+kernel void tint_symbol() {
+  return;
+}
+