swift234

Fix the declaration of shim visibility on ELF platforms.

Declarations with protected visibility are assumed to be defined
within the current linkage unit, so we have to use default visibility
if we don't know that we're building that.  Teach the shim
visibility header to only use protected visibility when the
__SWIFT_CURRENT_DYLIB macro is defined, and define it when building
the standard library.  Eventually we should change
SWIFT_RUNTIME_STDLIB_INTERFACE and SWIFT_RUNTIME_EXPORT to be
parameterized by the defining dylib so that this works for all the
overlay stubs, too; for now, special-casing swiftCore is necessary
to fix the LInux build.