writer/hlsl: Simplify emission logic, clean up output

And fix issues where global variables would not be emitted unless they were transitively referenced by an entry point.

This change requires crbug.com/tint/697 to be fixed before landing.

Change-Id: I712bd9d369e08c9a3cdfb0f114c3609584f91f28
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54241
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/sem/type_mappings.h

@@ -26,6 +26,7 @@ class Expression;
 class Function;
 class MemberAccessorExpression;
 class Statement;
+class Struct;
 class StructMember;
 class Type;
 class TypeDecl;
@@ -59,6 +60,7 @@ struct TypeMappings {
   StructMember* operator()(ast::StructMember*);
   Type* operator()(ast::Type*);
   Type* operator()(ast::TypeDecl*);
+  Struct* operator()(ast::Struct*);
   Variable* operator()(ast::Variable*);
   //! @endcond
 };