[lldb] Extract more fields from NSException values

This patch teaches LLDB about more fields on NSException Obj-C objects, specifically we can now retrieve the "name" and "reason" of an NSException. The goal is to eventually be able to have SB API that can provide details about the currently thrown/caught/processed exception.

Differential Revision: https://reviews.llvm.org/D43884

llvm-svn: 346695