Opaque type metadata accessor functions could be miscompiled for functions that
contain `if #available` checks for inactive platforms. For example, this
function will always return `A` when compiled for macOS, but the opaque type
accessor would instead return the type metadata for `B`:
```
func f() -> some P {
if #available(iOS 99, *) {
return A() // Returns an A on macOS
} else {
return B()
}
}
```
Resolves rdar://139487970.
* [IRGen] Use ConditionalDominanceScope in AbstractMetadataAccessor::emit
rdar://103179745
Under certain conditions the missing dominance scope caused a cached witness table ref to be used in a block that it was not available in.
* Fix target in test
If particular opaque type descriptor has multiple conditionally available
underlying types, emit a special type and witness reference accessors that
would be called at runtime to determine actual underlying type.
