Move type base classes into type/ folder.

This CL moves sem/type and copies sem/node into the type/ folder. The
type subclasses are moved over to using type::Type while remaining in
the sem:: namespace. They will be moved over in followup CLs.

Bug: tint:1718
Change-Id: I3f3495328d734f88e4fc2dfbc6705343f1198dc5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113180
Reviewed-by: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/ast/expression.h

@@ -19,7 +19,6 @@
 #include <vector>
 
 #include "src/tint/ast/node.h"
-#include "src/tint/sem/type.h"
 
 namespace tint::ast {
 

src/tint/ast/module_clone_test.cc

@@ -136,7 +136,7 @@ const declaration_order_check_4 : i32 = 1;
     for (auto* src_node : src.ASTNodes().Objects()) {
         src_nodes.emplace(src_node);
     }
-    std::unordered_set<const sem::Type*> src_types;
+    std::unordered_set<const type::Type*> src_types;
     for (auto* src_type : src.Types()) {
         src_types.emplace(src_type);
     }