[clang] Fix emitVoidPtrVAArg for non-zero default alloca address space

[jrtc27]

Summary

Indirect arguments are passed on the stack and so va_arg should use the
default alloca address space, not hard-code 0, for pointers to those.
The only in-tree target with a non-zero default alloca address space is
AMDGPU, but that does not support variadic arguments, so we cannot test
this upstream. However, downstream in CHERI LLVM (and Morello LLVM, a
further fork of that) we have targets that do both and so require this
change.

clang/lib/CodeGen/TargetInfo.cpp

if (IsIndirect) {
     DirectAlign = CGF.getPointerAlign();
   } else {
     DirectSize = ValueInfo.Width;
     DirectAlign = ValueInfo.Align;
   }
 
   // Cast the address we've calculated to the right type.
   llvm::Type *DirectTy = CGF.ConvertTypeForMem(ValueTy), *ElementTy = DirectTy;
-  if (IsIndirect)
-    DirectTy = DirectTy->getPointerTo(0);
+  if (IsIndirect) {
+    unsigned AllocaAS = CGF.CGM.getDataLayout().getAllocaAddrSpace();
+    DirectTy = DirectTy->getPointerTo(AllocaAS);
+  }
 
   Address Addr = emitVoidPtrDirectVAArg(CGF, VAListAddr, DirectTy, DirectSize,
                                         DirectAlign, SlotSizeAndAlign,
                                         AllowHigherAlign, ForceRightAdjust);
 
   if (IsIndirect) {
     Addr = Address(CGF.Builder.CreateLoad(Addr), ElementTy, ValueInfo.Align);
   }