Convert @id to an expression.

This CL updates the @id AST nodes to store an expression instead
of a literal value. This is in anticipation of the parser updates
for ID expressions.

Bug: tint:1633
Change-Id: I4dd626007dd1f9093999a0236e220ffdbc9e62db
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/100760
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/resolver/variable_validation_test.cc b/src/tint/resolver/variable_validation_test.cc
index 3416909..9843d2f 100644
--- a/src/tint/resolver/variable_validation_test.cc
+++ b/src/tint/resolver/variable_validation_test.cc
@@ -98,7 +98,7 @@
     // ...
     // @id(N) override oN : i32;
     constexpr size_t kLimit = std::numeric_limits<decltype(OverrideId::value)>::max();
-    Override("reserved", ty.i32(), Id(kLimit));
+    Override("reserved", ty.i32(), Id(AInt(kLimit)));
     for (size_t i = 0; i < kLimit; i++) {
         Override("o" + std::to_string(i), ty.i32());
     }