[clang][ConstExprEmitter] handle NullToPointer ImplicitCastExpr

Consider the following statement:
    void* foo = ((void *)0);

For the sub-AST:
    | `-ImplicitCastExpr 'const void *' <NullToPointer>
    |   `-CStyleCastExpr 'void *' <NullToPointer>
    |     `-IntegerLiteral 'int' 0

If the subexpression of the cast is itself the NULL constant, then
ImplicitCastExpr should emit the NULL pointer constant.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D156175

Author: nickdesaulniers

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1131,6 +1131,10 @@ class ConstExprEmitter :
       if (const auto *S = dyn_cast<StringLiteral>(subExpr))
         return CGM.GetAddrOfConstantStringFromLiteral(S).getPointer();
       return nullptr;
+    case CK_NullToPointer:
+      if (llvm::Constant *C = Visit(subExpr, destType))
+        return CGM.EmitNullConstant(destType);
+      return nullptr;
 
     case CK_IntToOCLSampler:
       llvm_unreachable("global sampler variables are not generated");
@@ -1187,7 +1191,6 @@ class ConstExprEmitter :
     case CK_IntegralComplexToFloatingComplex:
     case CK_PointerToIntegral:
     case CK_PointerToBoolean:
-    case CK_NullToPointer:
     case CK_IntegralCast:
     case CK_BooleanToSignedIntegral:
     case CK_IntegralToPointer: