[lldb] Support shared cache relative objc method types (#163663)
Support the types and name field in the relative method list to be
relative to a buffer in the shared cache, not relative to the field in
the method list itself.
A new magic bit, 0x20000000, is attached to method lists where the types
are encoded in this way. This is covered by the existing tests when
running against a shared cache that uses this encoding.
rdar://147545351