[clang][Interp] Ignore ArrayDecay ops for null pointers

Just don't do anything and let later operations handle the diagnostics.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1933,7 +1933,7 @@ inline bool ArrayElemPop(InterpState &S, CodePtr OpPC, uint32_t Index) {
 inline bool ArrayDecay(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
 
-  if (Ptr.isDummy()) {
+  if (Ptr.isZero() || Ptr.isDummy()) {
     S.Stk.push<Pointer>(Ptr);
     return true;
   }

clang/test/AST/Interp/cxx11.cpp

@@ -28,3 +28,5 @@ struct S {
 };
 constexpr S s = { 5 };
 constexpr const int *p = &s.m + 1;
+
+constexpr const int *np2 = &(*(int(*)[4])nullptr)[0]; // ok