Parsing Instance Connects Broken

Nobody has claimed this yet.

Connections involving instances fail to parse, but work in the Scala FIRRTL Compiler.

The following currently fails:
```scala
circuit Instance:
module Sub:
output a: UInt<1>
a <= UInt<1>(0)

module Instance:
output a: {a: UInt<1>}

inst sub of Sub
a <= sub
a <- sub
```

The error is:

```
# circt-translate --import-firrtl new/Instance.fir
new/Instance.fir:11:5: error: expected '.' in field reference
a <- sub
^
```

Glancing at the parser, it looks like it expects `sub` to be in the value table, but it isn't. [There is special logic to handle `sub is invalid`](https://github.com/llvm/circt/blob/main/lib/Dialect/FIRRTL/Import/FIRParser.cpp#L1187), but that needs to be extended to handle connections to submodules. If this isn't a `sub is invalid` situation, then the parser expects to see a subfield.

Reproduce the failure with circt-translate --import-firrtl using the FIRRTL example in the issue. Inspect lib/Dialect/FIRRTL/Import/FIRParser.cpp around the existing “sub is invalid” handling at line 1187, then verify that connections to the submodule parse successfully without the expected-field-reference error.

Written by the indexing model from the issue text.