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>

test/samples/function.wgsl.expected.wgsl

@@ -2,6 +2,6 @@ fn main() -> f32 {
   return (((2.0 * 3.0) - 4.0) / 5.0);
 }
 
-[[stage(compute), workgroup_size(2, 1, 1)]]
+[[stage(compute), workgroup_size(2)]]
 fn ep() {
 }