PrintAsClang: Forward reference enums when used transitively

There are two main scenarios when printing a compatibility header that
references a @cdecl enum defined in Swift code. (1) When defined in the
same module as it's used we can print the definition normally and then
reference it. (2) When used in a different mode we need to print a
forward declaration before we can reference it.

This change adds printing the forward declaration and fix an issue where
the compiler would instead print an @include of the Swift module. The
import of the Swift module would work only in a local scenario where a
compatibility header and module would be generated under the same name.
However for a distributed frameworks we do not distribute the
compatibility header so this strategy doesn't work. Relying on a forward
declaration should be more reliable in all cases but clients may need to
import the other compatibility header explicitly.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -457,6 +457,14 @@ public:
       }
     }
 
+    if (isa<EnumDecl>(D) && !D->hasClangNode() &&
+        outputLangMode != OutputLanguageMode::Cxx) {
+      // We don't want to add an import for a @cdecl or @objc enum declared
+      // in Swift. We either do nothing for special enums like Optional as
+      // done in the prologue here, or we forward declare them.
+      return false;
+    }
+
     imports.insert(otherModule);
     return true;
   }
@@ -530,16 +538,20 @@ public:
   }
 
   void forwardDeclare(const EnumDecl *ED) {
-    // Don't forward declare C enums.
-    if (ED->getAttrs().getAttribute<CDeclAttr>())
-      return;
-
-    assert(ED->isObjC() || ED->hasClangNode());
+    assert(ED->isObjC() || ED->getAttrs().getAttribute<CDeclAttr>() ||
+           ED->hasClangNode());
     
     forwardDeclare(ED, [&]{
-      os << "enum " << getNameForObjC(ED) << " : ";
-      printer.print(ED->getRawType());
-      os << ";\n";
+      if (ED->getASTContext().LangOpts.hasFeature(Feature::CDecl)) {
+        // Forward declare in a way to be compatible with older C standards.
+        os << "typedef SWIFT_ENUM_FWD_DECL(";
+        printer.print(ED->getRawType());
+        os << ", " << getNameForObjC(ED) << ")\n";
+      } else {
+        os << "enum " << getNameForObjC(ED) << " : ";
+        printer.print(ED->getRawType());
+        os << ";\n";
+      }
     });
   }
 
@@ -609,6 +621,7 @@ public:
     } else if (addImport(TD)) {
       return;
     } else if (auto ED = dyn_cast<EnumDecl>(TD)) {
+      // Treat this after addImport to filter out special enums from the stdlib.
       forwardDeclare(ED);
     } else if (isa<GenericTypeParamDecl>(TD)) {
       llvm_unreachable("should not see generic parameters here");