Rountrip PDBs with missing constant signature

**Repro**

Program.cs:
```C#
using System;

class Program
{
static void Main(string[] args)
{
const float f = 1.0f;
double x = f;
Console.WriteLine(x);
}
}
```

1. `csc Program.cs /debug:portable`
2. `pdb2pdb a.exe`
3. `pdb2pdb a.exe /pdb a.pdb2 /out a.pdb3`
```
PDB0017: token 0x06000001: Invalid local constant data
```

The second step produces a PDB with NIL constant token signature since there is no StandAloneSig in a.exe that matches `float` signature.

The third step then calls ISymUnmanagedConstant.GetSignature on such constant and reports an error.
We could infer the constant signature from the value for primitive types.