The Problem:

Problem observed when being used in coverlet:
coverlet-coverage/coverlet#1471
If an exception caused `NativePdbWriter.Write` to never be
called, it would not call `SymWriter.Close`,
which in turn meant `Marshal.ReleaseComObject` was left uncalled.

The garbage collector will eventually destroy the object
and thereby release the COM object, but that happens non-deterministically.
The COM object is holding to a file handle, which prevents
other operations on the written file until it is released.
The result was random file access issues.

The Solution:

Luckily NativePdbWriter is IDisposable. I added a call to
`writer.Close` there. But now it could be called twice,
so I had to add a boolean to SymWriter to avoid releasing
everything twice.