swift234

SIL: Round-trip DynamicSelfType properly

When DynamicSelfType occurs outside of a class body (for example,
inside of a SIL function), it is not enough to simply utter 'Self',
because then we lose the underlying type.

Instead, print it out as '@dynamic_self Foo', where 'Foo' is the
underlying class type or archetype, and add parser support for
the same.

Fixes <rdar://problem/27735857>.