Make sure GetObjectDescription falls back to the Objective-C runtime.

This fixes an unintended regression introduced by
https://reviews.llvm.org/D61451 by making sure the Objective-C runtime
is also tried when the "correct" language runtime failed to return an
object description.

rdar://problem/50791055

Differential Revision: https://reviews.llvm.org/D62015

llvm-svn: 360929