AST: support ObjC lifetime qualifiers in MS ABI

Adjust the ObjC protocol conformance workaround to be more extensible.
Use a synthetic type for the protocol (`struct Protocol`).  Embed this
within a reserved namespace to permit extending the extended pointer
type qualifiers similarly for ObjC lifetime qualifiers.

Introduce additional special handling for `__autoreleasing`, `__strong`,
and `__weak` Objective C lifetime qualifiers.  We decorate these by
creating an artificial template type `Autoreleasing`, `Strong`, or
`Weak` in the `__ObjC` namespace.  These are only considered in the
template type specialization and not the function parameter.

llvm-svn: 324701