Resolver: Generate mapping for ast::Type -> sem::Type

Currently untestable as nothing currently calls Type(const ast::Type* ty).

Bug: tint:724
Change-Id: I92dd772acd758b0960a7e9a19f15c91414ab505c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49527
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/sem/info.h

@@ -88,7 +88,9 @@ class Info {
   }
 
  private:
-  std::unordered_map<const CastableBase*, const sem::Node*> map;
+  // 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;
 };
 
 }  // namespace sem

src/sem/type_mappings.h

@@ -27,6 +27,7 @@ class Function;
 class MemberAccessorExpression;
 class Statement;
 class StructMember;
+class Type;
 class Variable;
 }  // namespace ast
 
@@ -42,6 +43,7 @@ class Statement;
 class Struct;
 class StructType;
 class StructMember;
+class Type;
 class Variable;
 
 /// TypeMappings is a struct that holds undefined `operator()` methods that's
@@ -58,6 +60,7 @@ struct TypeMappings {
   Statement* operator()(ast::Statement*);
   Struct* operator()(sem::StructType*);
   StructMember* operator()(ast::StructMember*);
+  Type* operator()(ast::Type*);
   Variable* operator()(ast::Variable*);
   //! @endcond
 };