[clang][Interp] Fix a crash when calling invalid constexpr functions

[aaron.ballman]

LGTM

clang/test/AST/Interp/functions.cpp

210–212

It'd be nice if we didn't issue duplicate notes in this case, but given that the behavior matches the reference implementation, it's reasonable enough.

clang/lib/AST/Interp/InterpFrame.cpp

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.
+  if (!Levels.empty())
     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()) {

clang/test/AST/Interp/functions.cpp

constexpr int second() const {
     return 1;
   }
 };
 
 constexpr int nyd(int m);
 constexpr int doit() { return nyd(10); }
 constexpr int nyd(int m) { return m; }
 static_assert(doit() == 10, "");
+
+namespace InvalidCall {
+  struct S {
+    constexpr int a() const { // expected-error {{never produces a constant expression}} \
+                              // ref-error {{never produces a constant expression}}
+      return 1 / 0; // expected-note 2{{division by zero}} \
+                    // expected-warning {{is undefined}} \
+                    // ref-note 2{{division by zero}} \
+                    // ref-warning {{is undefined}}
+    }
+  };
+  constexpr S s;
+  static_assert(s.a() == 1); // expected-error {{not an integral constant expression}} \
+                             // expected-note {{in call to}} \
+                             // ref-error {{not an integral constant expression}} \
+                             // ref-note {{in call to}}
+}