[FIRRTL] Don't allow annotations to target enumeration variants

I don't think that it makes sense for annotations to target a variant of an enumeration type as if it were a regular aggregate. In the following example, the annotation should not be able to target the `X` variant.
```firrtl
circuit Foo: %[[
{
"class":"firrtl.transforms.DontTouchAnnotation",
"target":"~Foo|Foo>w.X"
}
]]
module Foo:
wire w : {| X |}
w is invalid
```

Right now, `firtool` gives the following error message:
```
./test.fir:8:5: error: field access 'X' into non-bundle type ''<>''
wire w : {| X |}
^
./test.fir:8:5: note: see current operation: %0 = "firrtl.wire"() {annotations = [], name = "w", nameKind = #firrtl} : () -> !firrtl.enum>
./test.fir:1:1: error: Unable to resolve target of annotation: {class = "firrtl.transforms.DontTouchAnnotation", target = "~Foo|Foo>w.X"}
circuit Foo: %[[
^
```

Start by reproducing the FIRRTL example with firtool and inspect how annotation targets resolve field access on enumeration types. The change is done when an annotation cannot target the X enumeration variant as though it were an aggregate, with an appropriate diagnostic and no misleading field-access failure.

Written by the indexing model from the issue text.