swift234

stdlib: COFF variant of "visibility" macro

Currently, LLVM supports the current three common object file formats (COFF,
ELF, MachO).  Unfortunately, COFF does not provide a compiler macro to identify
the object file format.  If neither `__MACH__` nor `__ELF__` is defined, then
assume that the object file format being used is COFF.  Within the COFF target
handling, do not use `__declspec(dllexport)` for cygwin targets.