[FIRRTL] Layers and Analog

Consider:

FIRRTL version 4.0.0

circuit Foo :
  layer L, bind :

  extmodule Bar :
    input a : Analog<1>
  public module Foo :
    input a : Analog<1>

    layerblock L:
      inst b of Bar
      attach(b.a, a)

Presently this fails in the verifier after LowerLayers:

// -----// IR Dump Before LowerLayers (firrtl-lower-layers) //----- //
firrtl.circuit "Foo" {
  firrtl.layer @L bind {
  }
  firrtl.extmodule private @Bar(in a: !firrtl.analog<1>) attributes {convention = #firrtl<convention scalarized>}
  firrtl.module @Foo(in %a: !firrtl.analog<1>) attributes {convention = #firrtl<convention scalarized>} {
    firrtl.layerblock @L {
      %b_a = firrtl.instance b @Bar(in a: !firrtl.analog<1>)
      firrtl.attach %b_a, %a : !firrtl.analog<1>, !firrtl.analog<1>
    }
  }
}

analog-layer.fir:13:7: error: analog types may not be connected
      attach(b.a, a)
      ^
analog-layer.fir:13:7: note: see current operation: "firrtl.matchingconnect"(%0, %arg0) : (!firrtl.analog<1>, !firrtl.analog<1>) -> ()

LowerLayers should diagnose this if unsupported, or learn to emit an appropriate connect operation (attach).

Since attach is bidirectional, it seems like it should not be allowed across layer boundaries for consistency with connect in that direction.

Start with the LowerLayers pass and the analog-layer.fir example shown in the report; trace why the lowered layerblock produces a matchingconnect for Analog values. Done means LowerLayers either emits an appropriate attach operation or gives a clear diagnostic when the connection is unsupported, with the layer-boundary behavior covered by a test.