Change ModuleList::GetSharedModule so that it will reject "stub

libraries" altogether.  On Mac/iOS, these are libraries which have
a UUID and nlist records but no text or data.  If one of these
gets into the global module list, every time we try to search
for a given filename/arch/UUID, we'll get this stub library back.
We need to prevent them from getting added to the module list
altogether.

I thought about doing this down in ObjectFileMachO -- just rejecting
the file as a valid binary file altogether -- but Greg didn't want
to take that hard line approach at this point, he wanted to keep
the ability for lldb to read one of these if someone wanted to in
the future.

<rdar://problem/23035075> 

llvm-svn: 250979