[Generated header] Emit members of enum extensions into C++ class

As we do with Swift structs, emit the members of extensions of Swift
enums into the corresponding C++ class. This includes exposing more of
the Optional API from the standard library into Swift.

Author: DougGregor

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -882,6 +882,14 @@ class DeclAndTypePrinter::Implementation
           os << "\n";
 
           printMembers(ED->getMembers());
+
+          for (const auto *ext :
+               owningPrinter.interopContext.getExtensionsForNominalType(ED)) {
+            if (!cxx_translation::isExposableToCxx(ext->getGenericSignature()))
+              continue;
+
+            printMembers(ext->getMembers());
+          }
         },
         owningPrinter);
     recordEmittedDeclInCurrentCxxLexicalScope(ED);
@@ -2782,13 +2790,13 @@ static bool isStringNestedType(const ValueDecl *VD, StringRef Typename) {
              VD->getASTContext().getStringDecl();
 }
 
-static bool hasExposeAttr(const ValueDecl *VD, bool isExtension = false) {
+static bool hasExposeAttr(const ValueDecl *VD) {
   if (isa<NominalTypeDecl>(VD) && VD->getModuleContext()->isStdlibModule()) {
     if (VD == VD->getASTContext().getStringDecl())
       return true;
     if (VD == VD->getASTContext().getArrayDecl())
       return true;
-    if (VD == VD->getASTContext().getOptionalDecl() && !isExtension)
+    if (VD == VD->getASTContext().getOptionalDecl())
       return true;
     if (isStringNestedType(VD, "UTF8View") || isStringNestedType(VD, "Index"))
       return true;
@@ -2825,7 +2833,7 @@ static bool hasExposeAttr(const ValueDecl *VD, bool isExtension = false) {
         return false;
     }
 
-    return hasExposeAttr(ED->getExtendedNominal(), /*isExtension=*/true);
+    return hasExposeAttr(ED->getExtendedNominal());
   }
   return false;
 }

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -750,7 +750,7 @@ class ModuleWriter {
       for (const Decl *D : declsToWrite) {
         if (auto *ED = dyn_cast<ExtensionDecl>(D)) {
           const auto *type = ED->getExtendedNominal();
-          if (isa<StructDecl>(type))
+          if (isa<StructDecl>(type) || isa<EnumDecl>(type))
             printer.getInteropContext().recordExtensions(type, ED);
         }
       }

test/Interop/SwiftToCxx/enums/enum-associated-value-class-type-cxx.swift

@@ -14,6 +14,18 @@ public enum E {
     case i(Int)
 }
 
+extension E {
+  public func matchesIntValue(_ value: Int) -> Bool {
+    switch self {
+    case .c:
+      return false
+
+    case .i(let mine):
+      return mine == value
+    }
+  }
+}
+
 // CHECK:      SWIFT_INLINE_THUNK E E::_impl_c::operator()(const C& val) const {
 // CHECK-NEXT:   auto result = E::_make();
 // CHECK-NEXT:   auto op = swift::_impl::_impl_RefCountedClass::copyOpaquePointer(val);
@@ -29,3 +41,6 @@ public enum E {
 // CHECK-NEXT:   char * _Nonnull payloadFromDestruction = thisCopy->_destructiveProjectEnumData();
 // CHECK-NEXT:   return swift::_impl::implClassFor<C>::type::makeRetained(*reinterpret_cast<void **>(payloadFromDestruction));
 // CHECK-NEXT: }
+
+// CHECK: SWIFT_INLINE_THUNK bool E::matchesIntValue(swift::Int value) const {
+// CHECK-NEXT: return _impl::$s5Enums1EO15matchesIntValueySbSiF(value, _impl::swift_interop_passDirect_Enums_uint64_t_0_8_uint8_t_8_9(_getOpaquePointer()));

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -66,7 +66,8 @@
 // CHECK-NEXT: };
 // CHECK: SWIFT_INLINE_THUNK bool isSome() const;
 // CHECK: SWIFT_INLINE_THUNK bool isNone() const;
-// CHECK: SWIFT_INLINE_THUNK T_0_0 getUnsafelyUnwrapped() const SWIFT_SYMBOL({{.*}});
+// CHECK-DAG: SWIFT_INLINE_THUNK T_0_0 getUnsafelyUnwrapped() const SWIFT_SYMBOL({{.*}});
+// CHECK-DAG: SWIFT_INLINE_THUNK String getDebugDescription() const SWIFT_SYMBOL("s:SqslE16debugDescriptionSSvp");
 
 // CHECK: class SWIFT_SYMBOL({{.*}}) String final {
 // CHECK-NEXT: public: