Fix an issue where LLDB would detect an empty shared cache - which is...

Fix an issue where LLDB would detect an empty shared cache - which is legitimate albeit suboptimal - and warn about being unable to fetch ObjC class information, even though class data was actually properly loaded from the dynamic hashmap

Only ever warn about missing ObjC runtime class data if one either can't run the expressions to obtain such data, or the total count of classes is below a threshold that makes things sound really suspicious

Fixes rdar://27438500

llvm-svn: 276220