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.

Author: compnerd

stdlib/public/SwiftShims/Visibility.h

@@ -59,11 +59,16 @@
 # define SWIFT_RUNTIME_EXPORT __attribute__((__visibility__("default")))
 #endif
 
-#elif __CYGWIN__
-# define SWIFT_RUNTIME_EXPORT 
 #else
-// __dllexport/__dllimport for Windows?
-# error "Unimplemented object format"
+# if defined(__CYGWIN__)
+#  define SWIFT_RUNTIME_EXPORT
+# else
+#  if defined(__SWIFT_CURRENT_DYLIB)
+#   define SWIFT_RUNTIME_EXPORT __declspec(dllexport)
+#  else
+#   define SWIFT_RUNTIME_EXPORT
+#  endif
+# endif
 #endif
 
 /// Attribute for runtime-stdlib SPI interfaces.