Existing signature files become unusable

When a project has a signature file like:

```fsharp
val foo01 : int -> string -> _
val bar01 : int -> int -> _
```

They will be override by `--allsigs` to

```fsharp
val foo01: int -> string -> '?17893
val bar01: int -> int -> '?17894
```

**Repro steps**

Provide the steps required to reproduce the problem:

1. `Unzip`
2. `dotnet build`
3. `dotnet build --no-incremental`

[InvalidSignatureGeneration.zip](https://github.com/dotnet/fsharp/files/10665305/InvalidSignatureGeneration.zip)

**Expected behavior**

The project should still compile after the rebuild.

**Actual behavior**

Project no longer builds.

**Related information**

Detected via https://github.com/dotnet/fsharp/pull/14703