MixedVec Doesn't Work with SuggestName

As reported by @oharboe, a `MixedVec` doesn't seem to work when using the `.suggestName` to change the name of the numeric identifiers created by the `MixedVec`.

Consider:

```scala
import chisel3._
import chisel3.util.MixedVec
import circt.stage.ChiselStage

class Foo extends Module {
val a = IO(Input(MixedVec(Bool())))

a(0).suggestName("a")
}

println(ChiselStage.emitCHIRRTL(new Foo))
```

This produces the following CHIRRTL:

```
circuit Foo :
module Foo :
input clock : Clock
input reset : UInt<1>
input a : { 0 : UInt<1>} @[src/main/scala/main.scala 8:13]

skip
```

I'd expect to see:

```
input a : { a : UInt<1>}
```

Start with the MixedVec and suggestName behavior exercised by the ChiselStage.emitCHIRRTL reproducer in the issue. Run the example and trace CHIRRTL emission for the MixedVec field name; done means the generated input uses `a` instead of the numeric identifier `0`.

Written by the indexing model from the issue text.