Extra safeguards to ensure that we never query

Extra safeguards to ensure that we never query
the runtime if we have complete debug information
for a class.

Also made the Objective-C language runtime return
NULL when asked for the complete debug information
(i.e., information from DWARF, not information from
the runtime) if that information isn't present.  It
used to return a non-authoritative version, which
made it hard for clients to determine whether
complete debug information was available.

<rdar://problem/12608895>

llvm-svn: 167299