AST: Support @cdecl enums without an explicit C name

Accept `@cdecl` enums without an explicit C name. The C name defaults to
the Swift one. It is printed using the `SWIFT_ENUM` macro instead of
`SWIFT_ENUM_NAMED`.

Author: xymus

lib/AST/SwiftNameTranslation.cpp

@@ -50,7 +50,8 @@ getNameForObjC(const ValueDecl *VD, CustomNamesOnly_t customNamesOnly) {
   }
 
   if (auto cdeclAttr = VD->getAttrs().getAttribute<CDeclAttr>())
-    return cdeclAttr->Name;
+    if (!customNamesOnly || !cdeclAttr->Name.empty())
+      return VD->getCDeclName();
 
   if (customNamesOnly)
     return StringRef();

test/PrintAsObjC/cdecl-enums.swift

@@ -31,18 +31,18 @@ import Foundation
 // CHECK-NEXT:   ObjcEnumNamedHelloDolly = 4,
 // CHECK-NEXT: };
 
-@cdecl("ObjcEnumNamed") enum EnumNamed: Int {
+@cdecl(ObjcEnumNamed) enum EnumNamed: Int {
   case A, B, C, d, helloDolly
 }
 
-// CHECK-LABEL: typedef SWIFT_ENUM_NAMED(unsigned int, ExplicitValues, "ExplicitValues", closed) {
+// CHECK-LABEL: typedef SWIFT_ENUM(unsigned int, ExplicitValues, closed) {
 // CHECK-NEXT:   ExplicitValuesZim = 0,
 // CHECK-NEXT:   ExplicitValuesZang = 219,
 // CHECK-NEXT:   ExplicitValuesZung = 220,
 // CHECK-NEXT: };
 // NEGATIVE-NOT: ExplicitValuesDomain
 
-@cdecl("ExplicitValues") enum ExplicitValues: CUnsignedInt {
+@cdecl enum ExplicitValues: CUnsignedInt {
   case Zim, Zang = 219, Zung
 
   func methodNotExportedToC() {}

test/PrintAsObjC/cdecl-official.swift

@@ -28,7 +28,7 @@
 // CHECK: #endif
 
 // CHECK: /// Enums
-// CHECK: typedef SWIFT_ENUM_NAMED(int, CEnum, "CEnum", closed) {
+// CHECK: typedef SWIFT_ENUM(int, CEnum, closed) {
 // CHECK:   CEnumA = 0,
 // CHECK:   CEnumB = 1,
 // CHECK: };
@@ -84,7 +84,7 @@ func g_nullablePointers(_ x: UnsafeMutableRawPointer,
 
 /// Enums
 
-@cdecl("CEnum")
+@cdecl
 enum CEnum: CInt { case A, B }
 
 @cdecl("CEnumRenamed_CName")

test/attr/attr_cdecl_official.swift

@@ -49,6 +49,8 @@ enum SwiftEnum { case A, B }
 @cdecl("CEnum") enum CEnum: Int { case A, B }
 #endif
 
+@cdecl enum CEnumDefaultName: CInt { case A, B }
+
 @cdecl("CEnumNoRawType") enum CEnumNoRawType { case A, B }
 // expected-error @-1 {{'@cdecl' enum must declare an integer raw type}}