Changed lldb_private::Type over to use the intrusive ref counted pointers

so we don't have to lookup types in a type list by ID.

Changed the DWARF parser to remove the "can externally complete myself" bits
from the type when we are in the process of completing the type itself to
avoid an onslaught of external visible decl requests from the 
clang::ExternalASTSource.

llvm-svn: 142461