Support implicit _Atomic struct load / store

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>

Reviewers: dexonsmith

Subscribers: cfe-commits, efriedma, rsmith, aaron.ballman

Differential Revision: https://reviews.llvm.org/D49458

llvm-svn: 337410

clang/lib/Sema/Sema.cpp

@@ -481,6 +481,7 @@ ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
     case CK_ArrayToPointerDecay:
     case CK_FunctionToPointerDecay:
     case CK_ToVoid:
+    case CK_NonAtomicToAtomic:
       break;
     }
   }

clang/test/CodeGen/atomic-ops.c

@@ -183,6 +183,18 @@ 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