[NFC] [cxx-interop] Flatten findOptionSetEnum() and move it to ImportEnumInfo.cpp

It's at home there alongside other ObjC enum-specific logic, rather than
in the middle of ImportDecl.cpp (since it isn't directly or exclusively
related to importing decls).

(cherry picked from commit 5aa5bcf)

Author: j-hui

lib/ClangImporter/ImportDecl.cpp

@@ -846,26 +846,6 @@ static bool isPrintLikeMethod(DeclName name, const DeclContext *dc) {
 using MirroredMethodEntry =
   std::tuple<const clang::ObjCMethodDecl*, ProtocolDecl*, bool /*isAsync*/>;
 
-ImportedType importer::findOptionSetEnum(clang::QualType type,
-                                         ClangImporter::Implementation &Impl) {
-  if (auto typedefType = dyn_cast<clang::TypedefType>(type)) {
-    if (Impl.isUnavailableInSwift(typedefType->getDecl())) {
-      if (auto clangEnum =
-              findAnonymousEnumForTypedef(Impl.SwiftContext, typedefType)) {
-        // If this fails, it means that we need a stronger predicate for
-        // determining the relationship between an enum and typedef.
-        assert(
-            clangEnum.value()->getIntegerType()->getCanonicalTypeInternal() ==
-            typedefType->getCanonicalTypeInternal());
-        if (auto swiftEnum = Impl.importDecl(*clangEnum, Impl.CurrentVersion)) {
-          return {cast<TypeDecl>(swiftEnum)->getDeclaredInterfaceType(), false};
-        }
-      }
-    }
-  }
-  return {};
-}
-
 static bool areRecordFieldsComplete(const clang::CXXRecordDecl *decl) {
   for (const auto *f : decl->fields()) {
     auto *fieldRecord = f->getType()->getAsCXXRecordDecl();

lib/ClangImporter/ImportEnumInfo.cpp

@@ -247,6 +247,29 @@ const clang::Type *importer::getUnderlyingType(const clang::EnumDecl *decl) {
   return importer::desugarIfElaborated(decl->getIntegerType().getTypePtr());
 }
 
+ImportedType importer::findOptionSetEnum(clang::QualType type,
+                                         ClangImporter::Implementation &Impl) {
+  auto typedefType = dyn_cast<clang::TypedefType>(type);
+  if (!typedefType || !Impl.isUnavailableInSwift(typedefType->getDecl()))
+    // If this isn't a typedef, or it is a typedef that is available in Swift,
+    // then this definitely isn't used for {CF,NS}_OPTIONS.
+    return {};
+
+  auto clangEnum = findAnonymousEnumForTypedef(Impl.SwiftContext, typedefType);
+  if (!clangEnum)
+    return {};
+
+  // If this fails, it means that we need a stronger predicate for
+  // determining the relationship between an enum and typedef.
+  assert(clangEnum.value()->getIntegerType()->getCanonicalTypeInternal() ==
+         typedefType->getCanonicalTypeInternal());
+
+  if (auto *swiftEnum = Impl.importDecl(*clangEnum, Impl.CurrentVersion))
+    return {cast<TypeDecl>(swiftEnum)->getDeclaredInterfaceType(), false};
+
+  return {};
+}
+
 /// Determine the prefix to be stripped from the names of the enum constants
 /// within the given enum.
 void EnumInfo::determineConstantNamePrefix(const clang::EnumDecl *decl) {