[6.2][cxx-interop] Fix passing optional CoreFoundation types from Swift to C++

ExplanationThese types are passed around as reference counted pointers,
so the optional representation of them are nullable pointers.
Previously, we tried to wrap them in swift::Optional.
Issues: rdar://157667946
Original PRs: swiftlang#83609
Risk: Low, the fix is narrow.
Testing: Added a compiler test.
Reviewers: @hnrklssn

Author: Xazax-hun

lib/PrintAsClang/PrintClangFunction.cpp

@@ -622,7 +622,8 @@ static bool isOptionalForeignReferenceType(Type ty) {
   if (auto obj = ty->getOptionalObjectType()) {
     if (const auto *cd =
             dyn_cast_or_null<ClassDecl>(obj->getNominalOrBoundGenericNominal()))
-      return cd->isForeignReferenceType();
+      return cd->isForeignReferenceType() ||
+             cd->getForeignClassKind() == ClassDecl::ForeignKind::CFType;
   }
   return false;
 }
@@ -1780,7 +1781,8 @@ bool DeclAndTypeClangFunctionPrinter::hasKnownOptionalNullableCxxMapping(
         return typeInfo->canBeNullable;
       }
       if (const auto *cd = dyn_cast<ClassDecl>(nominal))
-        if (cd->isForeignReferenceType())
+        if (cd->isForeignReferenceType() ||
+            cd->getForeignClassKind() == ClassDecl::ForeignKind::CFType)
           return true;
       return isa_and_nonnull<clang::ObjCInterfaceDecl>(nominal->getClangDecl());
     }

test/Interop/SwiftToCxx/stdlib/core-foundation-types-in-cxx.swift

@@ -16,6 +16,12 @@ public func foobar(_ a: CFData) -> Bool {
     true
 }
 
+public func returnsCFDate() -> CFDate? {
+    nil
+}
+
+public func takesCFDate(x: CFDate?) {}
+
 public func networkThing() -> in_addr? {
     return nil
 }
@@ -25,4 +31,9 @@ public enum MyEnum {
 }
 
 // CHECK: SWIFT_EXTERN bool $s17UseCoreFoundation6foobarySbSo9CFDataRefaF(CFDataRef _Nonnull a) SWIFT_NOEXCEPT SWIFT_CALL; // foobar(_:)
+// CHECK: SWIFT_EXTERN CFDateRef _Nullable $s17UseCoreFoundation13returnsCFDateSo0E3RefaSgyF(void) SWIFT_NOEXCEPT SWIFT_CALL; // returnsCFDate()
+// CHECK: SWIFT_EXTERN void $s17UseCoreFoundation11takesCFDate1xySo0E3RefaSg_tF(CFDateRef _Nullable x) SWIFT_NOEXCEPT SWIFT_CALL; // takesCFDate(x:)
+
 // CHECK: SWIFT_INLINE_THUNK swift::Optional<in_addr> networkThing() noexcept SWIFT_SYMBOL("s:17UseCoreFoundation12networkThingSo7in_addrVSgyF") SWIFT_WARN_UNUSED_RESULT {
+// CHECK: SWIFT_INLINE_THUNK CFDateRef _Nullable returnsCFDate() noexcept SWIFT_SYMBOL("s:17UseCoreFoundation13returnsCFDateSo0E3RefaSgyF") SWIFT_WARN_UNUSED_RESULT {
+// CHECK: SWIFT_INLINE_THUNK void takesCFDate(CFDateRef _Nullable x) noexcept SWIFT_SYMBOL("s:17UseCoreFoundation11takesCFDate1xySo0E3RefaSg_tF") {