Support implicit _Atomic struct load / store

[jfb]

Summary

Using _Atomic to do implicit load / store is just a seq_cst atomic_load / atomic_store. Stores currently assert in Sema::ImpCastExprToType with 'can't implicitly cast lvalue to rvalue with this cast kind', but that's erroneous. The codegen is fine as the test shows.

While investigating I found that Richard had found the problem here: https://reviews.llvm.org/D46112#1113557

rdar://problem/40347123

[jfb]

I'm sure there are other bugs around _Atomic, please file a bug an CC me if that's the case. I'll commit this fix for now.

cfe/trunk/lib/Sema/Sema.cpp

if (VK == VK_RValue && !E->isRValue()) {
     switch (Kind) {
     default:
       llvm_unreachable("can't implicitly cast lvalue to rvalue with this cast "
                        "kind");
     case CK_LValueToRValue:
     case CK_ArrayToPointerDecay:
     case CK_FunctionToPointerDecay:
     case CK_ToVoid:
+    case CK_NonAtomicToAtomic:
       break;
     }
   }
   assert((VK == VK_RValue || !E->isRValue()) && "can't cast rvalue to lvalue");
 #endif
 
   diagnoseNullableToNonnullConversion(Ty, E->getType(), E->getLocStart());
   diagnoseZeroToNullptrConversion(Kind, E);

cfe/trunk/test/CodeGen/atomic-ops.c

 float ff3(_Atomic(float) *d) {
   return __c11_atomic_exchange(d, 2, memory_order_seq_cst);
 }
 
 struct S {
   double x;
 };
 
+void implicit_store(_Atomic(struct S) *a, struct S s) {
+  // CHECK-LABEL: @implicit_store(
+  // CHECK: store atomic i64 %{{.*}}, i64* %{{.*}} seq_cst, align 8
+  *a = s;
+}
+
+struct S implicit_load(_Atomic(struct S) *a) {
+  // CHECK-LABEL: @implicit_load(
+  // CHECK: load atomic i64, i64* %{{.*}} seq_cst, align 8
+  return *a;
+}
+
 struct S fd1(struct S *a) {
   // CHECK-LABEL: @fd1
   // CHECK: [[RETVAL:%.*]] = alloca %struct.S, align 4
   // CHECK: bitcast %struct.S* {{.*}} to i64*
   // CHECK: [[CAST:%.*]]  = bitcast %struct.S* [[RETVAL]] to i64*
   // CHECK: [[CALL:%.*]]   = call i64 @__atomic_load_8(
   // CHECK: store i64 [[CALL]], i64* [[CAST]], align 4
   struct S ret;