Atomic list index syntax is marked as NonAtomic

I'm not entirely sure but when I have the following code:

```fsharp
let y = [ 0; 2; 4 ][1]
```

I get
```fsharp
App
(NonAtomic, false,
ArrayOrListComputed
(false,
Sequential
(SuppressNeither, true,
Const (Int32 0, tmp.fsx (1,10--1,11)),
Sequential
(SuppressNeither, true,
Const (Int32 2, tmp.fsx (1,13--1,14)),
Const (Int32 4, tmp.fsx (1,16--1,17)),
tmp.fsx (1,13--1,17)), tmp.fsx (1,10--1,17)),
tmp.fsx (1,8--1,19)),
ArrayOrListComputed
(false, Const (Int32 1, tmp.fsx (1,20--1,21)),
tmp.fsx (1,19--1,22)), tmp.fsx (1,8--1,22))
```

**Expected behavior**

I was expecting `App(Atomic, false, ...)`

**Actual behavior**

The`SynExpr.App` contains `NonAtomic` while inside fsi I get:


**Known workarounds**

/

**Related information**

Provide any related information (optional):

FCS 41.0.0-preview.21472.3

//cc @dsyme