tint: Clean up legacy type cruft

Fix TODOs dating back to when types were an AST / SEM hybrid concept.
Bring the `arch.md` to reflect how things work today.

Bug: tint:724
Change-Id: I6bf4174158cf490f2839aeed78164b66e3410f27
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96141
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/sem/info.h

@@ -37,10 +37,9 @@ class Info {
 
     /// Resolves to the return type of the Get() method given the desired sementic
     /// type and AST type.
-    template <typename SEM, typename AST_OR_TYPE>
-    using GetResultType = std::conditional_t<std::is_same<SEM, InferFromAST>::value,
-                                             SemanticNodeTypeFor<AST_OR_TYPE>,
-                                             SEM>;
+    template <typename SEM, typename AST>
+    using GetResultType =
+        std::conditional_t<std::is_same<SEM, InferFromAST>::value, SemanticNodeTypeFor<AST>, SEM>;
 
     /// Constructor
     Info();
@@ -56,36 +55,36 @@ class Info {
     /// @return this Program
     Info& operator=(Info&& rhs);
 
-    /// Get looks up the semantic information for the AST or type node `node`.
-    /// @param node the AST or type node
+    /// Get looks up the semantic information for the AST node `node`.
+    /// @param ast_node the AST node
     /// @returns a pointer to the semantic node if found, otherwise nullptr
     template <typename SEM = InferFromAST,
-              typename AST_OR_TYPE = CastableBase,
-              typename RESULT = GetResultType<SEM, AST_OR_TYPE>>
-    const RESULT* Get(const AST_OR_TYPE* node) const {
-        auto it = map_.find(node);
+              typename AST = CastableBase,
+              typename RESULT = GetResultType<SEM, AST>>
+    const RESULT* Get(const AST* ast_node) const {
+        auto it = map_.find(ast_node);
         if (it == map_.end()) {
             return nullptr;
         }
         return As<RESULT>(it->second);
     }
 
-    /// Add registers the semantic node `sem_node` for the AST or type node `node`.
-    /// @param node the AST or type node
+    /// Add registers the semantic node `sem_node` for the AST node `node`.
+    /// @param ast_node the AST node
     /// @param sem_node the semantic node
-    template <typename AST_OR_TYPE>
-    void Add(const AST_OR_TYPE* node, const SemanticNodeTypeFor<AST_OR_TYPE>* sem_node) {
+    template <typename AST>
+    void Add(const AST* ast_node, const SemanticNodeTypeFor<AST>* sem_node) {
         // Check there's no semantic info already existing for the node
-        TINT_ASSERT(Semantic, Get(node) == nullptr);
-        map_.emplace(node, sem_node);
+        TINT_ASSERT(Semantic, Get(ast_node) == nullptr);
+        map_.emplace(ast_node, sem_node);
     }
 
-    /// Replace replaces any existing semantic node `sem_node` for the AST or type node `node`.
-    /// @param node the AST or type node
+    /// Replace replaces any existing semantic node `sem_node` for the AST node `node`.
+    /// @param ast_node the AST node
     /// @param sem_node the new semantic node
-    template <typename AST_OR_TYPE>
-    void Replace(const AST_OR_TYPE* node, const SemanticNodeTypeFor<AST_OR_TYPE>* sem_node) {
-        map_[node] = sem_node;
+    template <typename AST>
+    void Replace(const AST* ast_node, const SemanticNodeTypeFor<AST>* sem_node) {
+        map_[ast_node] = sem_node;
     }
 
     /// Wrap returns a new Info created with the contents of `inner`.
@@ -110,9 +109,8 @@ class Info {
     const sem::Module* Module() const { return module_; }
 
   private:
-    // TODO(crbug.com/tint/724): Once finished, this map should be:
-    // std::unordered_map<const ast::Node*, const sem::Node*>
-    std::unordered_map<const CastableBase*, const CastableBase*> map_;
+    // The map of AST node to semantic node
+    std::unordered_map<const ast::Node*, const sem::Node*> map_;
     // The semantic module
     sem::Module* module_ = nullptr;
 };