This replaces the '[volatile]' flag. Now, class_method and
super_method are only used for vtable dispatch.
The witness_method instruction is still overloaded for use
with both ObjC protocol requirements and Swift protocol
requirements; the next step is to make it only mean the
latter, also using objc_method for ObjC protocol calls.
This fixes a crash when referencing partially-applied methods
from @_inlineable functions.
Also, curry thunks for private methods do not need shared
linkage; private is sufficient.
