Upgrade the old mangling from a list of argument types to a
list of requiremnets. For now, only same-type requirements
may actually be mangled since those are all that are available
to the surface language.
Reconstruction of existential types now consists of demangling (a list of)
base protocol(s), decoding the constraints, and converting the same-type
constraints back into a list of arguments.
rdar://96088707
Update cast feasibility analysis to take parameterized protocols into account. The cast classifier was used to being able to essentially just run `conformsToProtocol` on the existential type, but for parameterized protocol types this will ignore the extra requirements imposed by the argument clause. For now, route everything through the runtime.
