[IRGen] Generate plausible signature for .cxx_{construct,destruct}

"@?" is not a method signature. An acceptable format is "vM@0:N", where
N is the pointer size and M is twice the pointer size. The numeric
values don't matter anymore on Apple platforms because the runtime does
not rely on them (they used to be stack offsets from the base pointer),
but given that the first (self) and second (_cmd) parameters are
pointers, the expected value is easily obtained and it is probably good
style to include it.

Fixes https://bugs.swift.org/browse/SR-8203

Author: fay59

lib/IRGen/GenObjC.cpp

@@ -1159,7 +1159,9 @@ void irgen::emitObjCMethodDescriptorParts(IRGenModule &IGM,
   /// The first element is the selector.
   selectorRef = IGM.getAddrOfObjCMethodName(selector.str());
 
-  /// The second element is the type @encoding.
+  /// The second element is the method signature. A method signature is made of
+  /// the return type @encoding and every parameter type @encoding, glued with
+  /// numbers that used to represent stack offsets for each of these elements.
   CanSILFunctionType methodType = getObjCMethodType(IGM, method);
   atEncoding = getObjCEncodingForMethodType(IGM, methodType, extendedEncoding);
 
@@ -1349,9 +1351,13 @@ void irgen::emitObjCIVarInitDestroyDescriptor(IRGenModule &IGM,
   Selector selector(declRef);
   auto selectorRef = IGM.getAddrOfObjCMethodName(selector.str());
 
-  /// The second element is the type @encoding, which is always "@?"
-  /// for a function type.
-  auto atEncoding = IGM.getAddrOfGlobalString("@?");
+  /// The second element is the method signature. A method signature is made of
+  /// the return type @encoding and every parameter type @encoding, glued with
+  /// numbers that used to represent stack offsets for each of these elements.
+  auto ptrSize = IGM.getPointerSize().getValue();
+  llvm::SmallString<8> signature;
+  signature = "v" + llvm::itostr(ptrSize * 2) + "@0:" + llvm::itostr(ptrSize);
+  auto atEncoding = IGM.getAddrOfGlobalString(signature);
 
   /// The third element is the method implementation pointer.
   auto impl = llvm::ConstantExpr::getBitCast(objcImpl, IGM.Int8PtrTy);