tint/transform: Move State to anonymous namespace

Copy/pasting this transform as a starting point for new transforms
causes the `State` classes to clash, and weird things happen. This
prevents that from happening.

Change-Id: Ia1c6b2b96e4d6375309aed535d7a87372b839792
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93880
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/transform/expand_compound_assignment.cc

@@ -44,6 +44,8 @@ bool ExpandCompoundAssignment::ShouldRun(const Program* program, const DataMap&)
     return false;
 }
 
+namespace {
+
 /// Internal class used to collect statement expansions during the transform.
 class State {
   private:
@@ -163,6 +165,8 @@ class State {
     }
 };
 
+}  // namespace
+
 void ExpandCompoundAssignment::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
     State state(ctx);
     for (auto* node : ctx.src->ASTNodes().Objects()) {