Add a new type of plugin: Language plugin

The Language plugin is menat to answer language-specific questions that are not bound to the existence of a process. Those are still the domain of the LanguageRuntime plugin

The Language plugin will, instead, answer questions such as providing language-specific data formatters or expression evaluation

At the moment, the interface is hollowed out, and empty do-nothing plugins have been setup for ObjC, C++ and ObjC++

llvm-svn: 246212