This is a fairly bulky patch, but a lot of it involves rearranging existing code
What it does:

- it introduces a concept of EncodingToType to the ObjCLanguageRuntime
  The ObjC runtime has a "type encoding" feature that describes types as strings
  The EncodingToType is a decoder for that format, making types out of type encoding strings
This feature already existed in some shape as we were using it to create method signatures out of the runtime, but this checkin extends the parser to support the full syntax, and moves things so that more parts of LLDB have access to this decoder

- it splits the ClassDescriptorV2 object to its own file, it was starting to grow too large

- it adds to the ClassDescriptor mechanism a notion of ivar storage; the ObjC runtime vends ivar information as well as method information
While ivar information is not ready for prime type (i.e. we don't want to add it to the runtime generated types for expression evaluator usage), there are potentially useful scenarios in which realizing ivar types could be useful. For now, the ClassDescriptor is going to hold ivar information directly. Existing code already allows describing ivars, this patch hooks those moving parts up so that one can actually ask a ClassDescriptor about ivars for the class it represents

and as a couple minor niceties:
- it makes it possible to retrieve the LLDB ClangASTContext that is associated to a clang::ASTContext
- it extends the ValueObject-to-ClassDescriptor API in the language runtime to deal correctly with base-class hierarchies