Global tweaks to handle having no sem::Type

Soon, we'll start migrating the AST from using sem::Types to ast::Types.
This change fixes up a bunch of places that makes the assumption that the semantic type is always expected.

Bug: tint:724
Change-Id: I96096bdf7177751ca6c6240e1739244cbeb82761
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49348
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/program_builder.h

@@ -861,6 +861,9 @@ class ProgramBuilder {
     /// @return either type or a pointer to a new ast::TypeName
     typ::Type MaybeCreateTypename(typ::Type type) const;
 
+    /// The ProgramBuilder
+    ProgramBuilder* const builder;
+
    private:
     /// CToAST<T> is specialized for various `T` types and each specialization
     /// contains a single static `get()` method for obtaining the corresponding
@@ -869,8 +872,6 @@ class ProgramBuilder {
     ///    `static typ::Type get(Types* t)`
     template <typename T>
     struct CToAST {};
-
-    ProgramBuilder* const builder;
   };
 
   //////////////////////////////////////////////////////////////////////////////