ast: Support non-literal workgroup_size parameters

Change the type of the values in an ast::WorkgroupDecoration to be
ast::Expression nodes, so that they can represent both
ast::ScalarExpression (literal) and ast::IdentifierExpression
(module-scope constant).

The Resolver processes these nodes to produce a uint32_t for the
default value on each dimension, and captures a reference to the
module-scope constant if it is overridable (which will soon be used by
the inspector and backends).

The WGSL parser now uses `primary_expression` to parse arguments to
workgroup_size.

Also added some WorkgroupSize() helpers to ProgramBuilder.

Bug: tint:713
Change-Id: I44b7b0021b925c84f25f65e26dc7da6b19ede508
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51262
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/inspector/inspector_test.cc

@@ -828,10 +828,8 @@ TEST_F(InspectorGetEntryPointTest, DefaultWorkgroupSize) {
 }
 
 TEST_F(InspectorGetEntryPointTest, NonDefaultWorkgroupSize) {
-  MakeEmptyBodyFunction("foo", ast::DecorationList{
-                                   Stage(ast::PipelineStage::kCompute),
-                                   create<ast::WorkgroupDecoration>(8u, 2u, 1u),
-                               });
+  MakeEmptyBodyFunction(
+      "foo", {Stage(ast::PipelineStage::kCompute), WorkgroupSize(8, 2, 1)});
 
   Inspector& inspector = Build();