PrintAsObjC: Add support for @_weakLinked attribute

Clang already takes availability into account when checking if a
declaration is weak imported, so this is only needed for testing.

Author: slavapestov

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -322,6 +322,13 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   void visitClassDecl(ClassDecl *CD) {
     printDocumentationComment(CD);
 
+    // This is just for testing, so we check explicitly for the attribute instead
+    // of asking if the class is weak imported. If the class has availablility,
+    // we'll print a SWIFT_AVAIALBLE() which implies __attribute__((weak_imported))
+    // already.
+    if (CD->getAttrs().hasAttribute<WeakLinkedAttr>())
+      os << "SWIFT_WEAK_IMPORT\n";
+
     bool hasResilientAncestry =
       CD->checkAncestry().contains(AncestryFlags::ResilientOther);
     if (hasResilientAncestry) {
@@ -2717,6 +2724,9 @@ class ModuleWriter {
            "#if !defined(SWIFT_AVAILABILITY)\n"
            "# define SWIFT_AVAILABILITY(plat, ...) __attribute__((availability(plat, __VA_ARGS__)))\n"
            "#endif\n"
+           "#if !defined(SWIFT_WEAK_IMPORT)\n"
+           "# define SWIFT_WEAK_IMPORT __attribute__((weak_import))\n"
+           "#endif\n"
            "#if !defined(SWIFT_DEPRECATED)\n"
            "# define SWIFT_DEPRECATED __attribute__((deprecated))\n"
            "#endif\n"

test/PrintAsObjC/classes.swift

@@ -805,3 +805,9 @@ public class NonObjCClass { }
   @objc func referenceSingleGenericClass(_: SingleImportedObjCGeneric<AnyObject>?) {}
 }
 // CHECK: @end
+
+// CHECK: SWIFT_WEAK_IMPORT
+// CHECK-NEXT: SWIFT_CLASS("_TtC7classes17WeakImportedClass")
+// CHECK-NEXT: @interface WeakImportedClass
+// CHECK-NEXT: @end
+@_weakLinked @objc class WeakImportedClass {}