Don't crash when passing &@selector to a _Nonnull parameter. Fixes PR24774.

The root cause here is that ObjCSelectorExpr is an rvalue, yet it can have its
address taken.  That's kind of awkward, but fixing this is awkward in other
ways, see https://llvm.org/bugs/show_bug.cgi?id=24774#c16 .  For now, just
fix the crash.

llvm-svn: 247740