tint: Add support for workgroupUniformLoad

Accept any type in the intrinsics definition, and then manually
validate that there are no atomics in the type. Add manual E2E tests
for composite types.

Use the BuiltinPolyfill transform to implement it for all backends.

Update the uniformity analysis with special-case tags for the builtin.

Fixed: tint:1780
Change-Id: I95786dff4df70a0b16ed1c53b853b5d0ec6bc501
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114862
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: James Price <jrprice@google.com>

test/tint/builtins/workgroupUniformLoad/array.wgsl

@@ -0,0 +1,5 @@
+var<workgroup> v : array<i32, 4>;
+
+fn foo() -> array<i32, 4> {
+  return workgroupUniformLoad(&v);
+}