[clang][Interp] Fix double-printing in InterpFrame::describe()

[tbaeder]

Summary

We add the pointer we want to print itself to the Levels array anyway, but then we print it once via printDesc(P.getDeclDesc()) and in the loop again via printDesc(It.getFieldDesc()).

[aaron.ballman]

LGTM

clang/lib/AST/Interp/InterpFrame.cpp

void print(llvm::raw_ostream &OS, const Pointer &P, ASTContext &Ctx,
   if (!Ty->isReferenceType())
     OS << "&";
   llvm::SmallVector<Pointer, 2> Levels;
   for (Pointer F = P; !F.isRoot(); ) {
     Levels.push_back(F);
     F = F.isArrayElement() ? F.getArray().expand() : F.getBase();
   }
 
+  // Drop the first pointer since we print it unconditionally anyway.
+  Levels.erase(Levels.begin());
+
   printDesc(P.getDeclDesc());
   for (const auto &It : Levels) {
     if (It.inArray()) {
       OS << "[" << It.expand().getIndex() << "]";
       continue;
     }
     if (auto Index = It.getIndex()) {
       OS << " + " << Index;

clang/test/AST/Interp/constexpr-nqueens.cpp

 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=ref %s
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -fexperimental-new-constant-interpreter -verify %s
 
 /// This is a version of the nqueens.cpp from SemaCXX/,
 /// but we don't initialize the State variable in the
 /// Board constructors.
 /// This tests that InterpFrame::describe().
 
-// FIXME: With the new interpreter, most of the described frames are
-//  currently broken in one way or another.
-
 typedef unsigned long uint64_t;
 
 struct Board {
   uint64_t State;
   bool Failed;
 
   constexpr Board() : Failed(false) {}
   constexpr Board(const Board &O) : Failed(O.Failed) {}