Issue Description:

Opaque result types are resilient types, but this should only matter when crossing a module boundary. When declaring and using opaque result types within a single module, this should work on all OSes as it should be a compile-time-only feature in this scenario.

Unfortunately, when I actually try this, I get a linker error.

func foo() -> some CustomStringConvertible {
    return 42
}
print(foo())

dyld: lazy symbol binding failed: Symbol not found: _swift_getOpaqueTypeMetadata
  Referenced from: /tmp/foo
  Expected in: /usr/lib/swift/libswiftCore.dylib

dyld: Symbol not found: _swift_getOpaqueTypeMetadata
  Referenced from: /tmp/foo
  Expected in: /usr/lib/swift/libswiftCore.dylib