[clang][Interp] Fix post-inc/dec operator result

[tbaeder]

Summary

We pushed the wrong value on the stack, always leaving a 0 behind.

[aaron.ballman]

LGTM!

clang/lib/AST/Interp/Interp.h

 };
 
 template <typename T, IncDecOp Op, PushVal DoPush>
 bool IncDecHelper(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   T Value = Ptr.deref<T>();
   T Result;
 
   if constexpr (DoPush == PushVal::Yes)
-    S.Stk.push<T>(Result);
+    S.Stk.push<T>(Value);
 
   if constexpr (Op == IncDecOp::Inc) {
     if (!T::increment(Value, &Result)) {
       Ptr.deref<T>() = Result;
       return true;
     }
   } else {
     if (!T::decrement(Value, &Result)) {

clang/test/AST/Interp/literals.cpp

namespace IncDec {
 
   constexpr int ptrDec1() { // expected-error {{never produces a constant expression}} \
                         // ref-error {{never produces a constant expression}}
     const int *p = arr;
     p -= 1;  // expected-note {{cannot refer to element -1 of array of 3 elements}} \
              // ref-note {{cannot refer to element -1 of array of 3 elements}}
     return *p;
   }
+
+  /// This used to leave a 0 on the stack instead of the previous
+  /// value of a.
+  constexpr int bug1Inc() {
+    int a = 3;
+    int b = a++;
+    return b;
+  }
+  static_assert(bug1Inc() == 3);
+
+  constexpr int bug1Dec() {
+    int a = 3;
+    int b = a--;
+    return b;
+  }
+  static_assert(bug1Dec() == 3);
+
 };
 #endif
 
 namespace CompoundLiterals {
   constexpr int get5() {
     return (int[]){1,2,3,4,5}[4];
   }
   static_assert(get5() == 5, "");