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/diagnostic/diagnostic.h

@@ -47,6 +47,7 @@ enum class System {
     Symbol,
     Test,
     Transform,
+    Type,
     Utils,
     Writer,
 };