If we're allowed to know at IRGen time what the underlying type of an opaque type is, we can
satisfy references to the opaque type's metadata or protocol witness tables by directly referencing
the underlying type instead.
Check the availability of decls that declare an opaque return type to ensure they deploy to a
runtime that supports opaque types.
rdar://problem/50731151
Fix a trio of issues involving mangling for opaque result types:
* Symbolic references to opaque type descriptors are not substitutions
* Mangle protocol extension contexts correctly
* Mangle generic arguments for opaque result types of generic functions
The (de-)serialization of generic parameter lists for opaque type
declarations is important for the last bullet, to ensure that the
mangling of generic arguments of opaque result types works across
module boundaries.
Fixes the rest of rdar://problem/50038754.
