Marker protocols don't exist at runtime, drop them when mangling a type
for the purposes of runtime type metadata or reflection. Fixes
rdar://82314404.
This improves source compatibility in cases where a marker
protocol is introduced after-the-fact (e.g., for `Error`),
and is safe because there's no runtime component to marker protocols.
There is no problem with having an existential value that involves a
marker protocol. However, since there is no runtime checking for
marker protocols, ban "as?" and "is" casts to them.
A marker protocol is a protocol with no requirements and with no ABI
footprint. They can be used to indicate semantics, only, and one can
never have values of a marker protocol type.
We still mangle marker protocols when they are used as generic
requirements, because they are a distinguishing characteristic for
overloading, but "no ABI footprint" means no protocol descriptors,
conformance descriptors, or dynamic discovery of any kind.
