Unconditionally support block introspection data in a new field at the end

of the block descriptor field.  This field is the ObjC style @encode
signature of the implementation function, and was to this point
conditionally provided in the block literal data structure.  That
provisional support is removed.

Additionally, eliminate unused enumerations for the block literal flags field.
The first shipping ABI unconditionally set (1<<29) but this bit is unused
by the runtime, so the second ABI will unconditionally have (1<<30) set so
that the runtime can in fact distinguish whether the additional data is
present or not.

llvm-svn: 96989