[CIR] Add support for delegating ctor with VTT (#1890)

This implements getting the VTT argument for a delegating constructor
and enables the corresponding test in delegating-ctor.cpp.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenClass.cpp

@@ -1536,7 +1536,8 @@ mlir::Value CIRGenFunction::GetVTTParameter(GlobalDecl GD, bool ForVirtualBase,
   uint64_t SubVTTIndex;
 
   if (Delegating) {
-    llvm_unreachable("NYI");
+    // If this is a delegating constructor call, just load the VTT.
+    return LoadCXXVTT();
   } else if (RD == Base) {
     // If the record matches the base, this is the complete ctor/dtor
     // variant calling the base variant in a class with virtual bases.