swift234

[Serialization] Fix a bug in the serialization of resilient functions

The SIL serializer can decide not to serialize the body of functions in the SIL
module, and only emit a declaration.  If we keep the original linkage kind
(public, private, etc) then the deserialized module won't pass verification
because we do not allow internal functions to have external declarations. This
commit changes the linkage kind for the functions that the serializer decides to
emit as a declaration (without a body).

rdar://21989088