swift234

IRGen: Emit and use accessors for generic type metadata

Instead of directly emitting calls to swift_getGenericMetadata*() and
referencing metadata templates, call a metadata accessor function
corresponding to the UnboundGenericType of the NominalTypeDecl.

The body of this accessor forwards arguments to a runtime metadata
instantiation function, together with the template.

Also, move some code around, so that metadata accesses which are
only done as part of the body of a metadata accessor function are
handled separately in emitTypeMetadataAccessFunction().

Apart from protocol conformances, this means metadata templates are
no longer referenced from outside the module where they were defined.