This gets adjustAccessLevelForProtocolExtension, a hack of sorts to
begin with, out of ValueDecl's general API, and down to a helper for
isAccessibleFrom and isSetterAccessibleFrom. (The only reason these
two don't go through access scopes is as an optimization.)
A protocol extension of a private protocol can define internal
or public members. We should not be able to find these members
from another file or module if an internal or public type
conforms to the protocol.
Fixes <rdar://problem/21380336>.
