Make an Error.h that declares the public ABI for errors.

NFC except that I added swift_errorRetain and swift_errorRelease
functions on non-ObjC targets so that we have consistent
functions to call in the runtime.  I have not changed everywhere
in the runtime to use these, nor have I changed the compiler to
call them.

stdlib/public/runtime/ErrorObjectNative.cpp

@@ -106,4 +106,12 @@ swift::swift_getErrorValue(const SwiftError *errorObject,
   out->errorConformance = errorObject->errorConformance;
 }
 
+SwiftError *swift::swift_errorRetain(SwiftError *error) {
+  return static_cast<SwiftError*>(swift_retain(error));
+}
+
+void swift::swift_errorRelease(SwiftError *error) {
+  swift_release(error);
+}
+
 #endif