[PrintAsObjC] Add '__attribute__((return))' for 'Never' return methods

* Add 'SWIFT_NORETURN' macro to the prologue. This macro is evaluated to
  '__attribute__((noreturn))' where supported.
* Apply 'SWIFT_NORETURN' to 'isUninhabited()' methods.

Author: rintaro

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -628,9 +628,10 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
       if (looksLikeInitMethod(AFD->getObjCSelector())) {
         os << " SWIFT_METHOD_FAMILY(none)";
       }
-      if (!methodTy->getResult()->isVoid() &&
-          !methodTy->getResult()->isUninhabited() &&
-          !AFD->getAttrs().hasAttribute<DiscardableResultAttr>()) {
+      if (methodTy->getResult()->isUninhabited()) {
+        os << " SWIFT_NORETURN";
+      } else if (!methodTy->getResult()->isVoid() &&
+                 !AFD->getAttrs().hasAttribute<DiscardableResultAttr>()) {
         os << " SWIFT_WARN_UNUSED_RESULT";
       }
     }
@@ -2337,6 +2338,11 @@ class ModuleWriter {
            "#else\n"
            "# define SWIFT_WARN_UNUSED_RESULT\n"
            "#endif\n"
+           "#if defined(__has_attribute) && __has_attribute(noreturn)\n"
+           "# define SWIFT_NORETURN __attribute__((noreturn))\n"
+           "#else\n"
+           "# define SWIFT_NORETURN\n"
+           "#endif\n"
            "#if !defined(SWIFT_CLASS_EXTRA)\n"
            "# define SWIFT_CLASS_EXTRA\n"
            "#endif\n"

test/PrintAsObjC/never.swift

@@ -12,5 +12,5 @@
 }
 
 // CHECK-LABEL: @interface NeverClass
-// CHECK:         - (void)doesNotReturn;
+// CHECK:         - (void)doesNotReturn SWIFT_NORETURN;
 // CHECK:       @end