[tint] Add subgroup builtin inputs

Adds parsing and SPIR-V codegen for subgroup_invocation_id and
subgroup_size.

Bug: tint:2000
Change-Id: Idcb9e2011c8c6129c5a0e5f0564048c14f8625a6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/143835
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/intrinsics.def b/src/tint/intrinsics.def
index 05ecb45..0774687 100644
--- a/src/tint/intrinsics.def
+++ b/src/tint/intrinsics.def
@@ -37,6 +37,8 @@
   num_workgroups
   sample_index
   sample_mask
+  subgroup_invocation_id
+  subgroup_size
   __point_size
 }