[clang][Interp] Fix converting function pointers to bool

[tbaeder]

Summary

Emit the proper PT_FnPtr instructions if we're handling function pointers here.

[aaron.ballman]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

case CK_IntegralCast: {
 
     if (FromT == ToT)
       return true;
 
     return this->emitCast(*FromT, *ToT, CE);
   }
 
   case CK_PointerToBoolean: {
+    PrimType PtrT = classifyPrim(SubExpr->getType());
+
     // Just emit p != nullptr for this.
     if (!this->visit(SubExpr))
       return false;
 
-    if (!this->emitNullPtr(CE))
+    if (!this->emitNull(PtrT, CE))
       return false;
 
-    return this->emitNEPtr(CE);
+    return this->emitNE(PtrT, CE);
   }
 
   case CK_ToVoid:
     return discard(SubExpr);
 
   default:
     assert(false && "Cast not implemented");
   }

clang/test/AST/Interp/functions.cpp

namespace FunctionReturnType {
 
   struct S {
     int i;
     void (*fp)();
   };
 
   constexpr S s{ 12 };
   static_assert(s.fp == nullptr, ""); // zero-initialized function pointer.
+
+  constexpr int (*op)(int, int) = add;
+  constexpr bool b = op;
+  static_assert(op, "");
+  static_assert(!!op, "");
+  constexpr int (*op2)(int, int) = nullptr;
+  static_assert(!op2, "");
 }
 
 namespace Comparison {
   void f(), g();
   constexpr void (*pf)() = &f, (*pg)() = &g;
 
   constexpr bool u13 = pf < pg; // ref-warning {{ordered comparison of function pointers}} \
                                 // ref-error {{must be initialized by a constant expression}} \