* let `SIL.Type` conform to `TypeProperties` to share the implementation of common type properties between the AST types and `SIL.Type`
* call references to an `AST.Type` `rawType` (instead of just `type`)
* remove unneeded stuff
* add comments
* factor out common methods of AST Type/CanonicalType into a `TypeProperties` protocol.
* add more APIs to AST Type/CanoncialType.
* move `MetatypeRepresentation` from SIL.Type to AST.Type and implement it with a swift enum.
* let `Builder.createMetatype` get a CanonicalType as instance type, because the instance type must not be a lowered type.
We need to keep the original linkage because it would be illegal to call a shared not-serialized function from a serialized function.
Also, rename the API to create the specialized function.
Generic specialization already takes care of removing metatype arguments of generic functions.
But sometimes non-generic functions have metatype arguments which must be removed.
We need handle this case with a function signature optimization.
This enables, for example, to use `OptionSet` in embedded swift.
rdar://121206953
