[clang][Interp][NFC] Add assertions to VM entry points

Assert that the previous call left the stack empty, as well as that
successful interpretations leave an empty stack.

clang/lib/AST/Interp/Context.cpp

@@ -27,6 +27,7 @@ Context::Context(ASTContext &Ctx) : Ctx(Ctx), P(new Program(*this)) {}
 Context::~Context() {}
 
 bool Context::isPotentialConstantExpr(State &Parent, const FunctionDecl *FD) {
+  assert(Stk.empty());
   Function *Func = P->getFunction(FD);
   if (!Func) {
     if (auto R = ByteCodeStmtGen<ByteCodeEmitter>(*this, *P).compileFunc(FD)) {
@@ -45,14 +46,28 @@ bool Context::isPotentialConstantExpr(State &Parent, const FunctionDecl *FD) {
 }
 
 bool Context::evaluateAsRValue(State &Parent, const Expr *E, APValue &Result) {
+  assert(Stk.empty());
   ByteCodeExprGen<EvalEmitter> C(*this, *P, Parent, Stk, Result);
-  return Check(Parent, C.interpretExpr(E));
+  if (Check(Parent, C.interpretExpr(E))) {
+    assert(Stk.empty());
+    return true;
+  }
+
+  Stk.clear();
+  return false;
 }
 
 bool Context::evaluateAsInitializer(State &Parent, const VarDecl *VD,
                                     APValue &Result) {
+  assert(Stk.empty());
   ByteCodeExprGen<EvalEmitter> C(*this, *P, Parent, Stk, Result);
-  return Check(Parent, C.interpretDecl(VD));
+  if (Check(Parent, C.interpretDecl(VD))) {
+    assert(Stk.empty());
+    return true;
+  }
+
+  Stk.clear();
+  return false;
 }
 
 const LangOptions &Context::getLangOpts() const { return Ctx.getLangOpts(); }

clang/lib/AST/Interp/InterpStack.h

@@ -75,6 +75,9 @@ public:
   /// Clears the stack without calling any destructors.
   void clear();
 
+  // Returns whether the stack is empty.
+  bool empty() const { return StackSize == 0; }
+
 private:
   /// All stack slots are aligned to the native pointer alignment for storage.
   /// The size of an object is rounded up to a pointer alignment multiple.