[Pitch] always export entry points in .swiftinterface files

If so, @kubamracek's Low-level linkage control pitch from a few weeks ago, and the discussion in that thread about representing the different symbols associated with declarations, is probably relevant here. Swift declarations in general are not necessarily associated with just one linker symbol; there can be many symbols for many different purposes that would have to be enumerated in order to provide full coverage of the possible symbols that would be needed in order to support interoperation with other languages.