[clang][Interp] Fix nullptr - nullptr expressions

They can happen and we used to run into an assertion.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1579,6 +1579,11 @@ inline bool SubPtr(InterpState &S, CodePtr OpPC) {
     return false;
   }
 
+  if (LHS.isZero() && RHS.isZero()) {
+    S.Stk.push<T>();
+    return true;
+  }
+
   T A = T::from(LHS.getIndex());
   T B = T::from(RHS.getIndex());
   return AddSubMulHelper<T, T::sub, std::minus>(S, OpPC, A.bitWidth(), A, B);

clang/test/AST/Interp/literals.cpp

@@ -1124,3 +1124,10 @@ namespace rdar8769025 {
     f1(0); // both-warning{{null passed to a callee that requires a non-null argument}}
   }
 }
+
+namespace nullptrsub {
+  void a() {
+    char *f = (char *)0;
+    f = (char *)((char *)0 - (char *)0);
+  }
+}