[FIRRTL] Instance Choices with Layers Error Out

I haven't thought through what should happen here, yet.

Consider the following FIRRTL which contains layerblocks under an instance choice:

``` firrtl
FIRRTL version 4.2.0
circuit Top :
option Target :
X

layer A, bind :

extmodule Foo :

module Bar enablelayer A :

public module Top enablelayer A :

instchoice divider of Foo, Target :
X => Bar

```

When compiled, this errors out with:

```
# firtool Foo.fir
Foo.fir:14:5: error: 'firrtl.instance_choice' op layers must be [@A], but got []
instchoice divider of Foo, Target :
^
Foo.fir:14:5: note: see current operation: "firrtl.instance_choice"() <{annotations = [], caseNames = [@Target::@X], layers = [], moduleNames = [@Foo, @Bar], name = "divider", nameKind = #firrtl, portAnnotations = [], portDirections = array, portNames = []}> : () -> ()
Foo.fir:10:3: note: original module declared here
module Bar enablelayer A :
```

Reproduce the example with firtool and inspect handling of the firrtl.instance_choice operation and its layer metadata. The issue does not define the intended behavior for layerblocks under an instance choice, so first establish that behavior; done means the intended case compiles without this error and the behavior is covered by verification.

Written by the indexing model from the issue text.