Use Never instead of Void as the fallback lock/mutex type.

[grynspan]

When using WASI without multithreading, or when using a platform to which Swift Testing has been incompletely ported, we don't know what type to use as a lock/mutex, so we use Void. However, turns out that produces a diagnostic:

warning: UnsafeMutablePointer has been replaced by UnsafeMutableRawPointer

So use Never instead. (Yes, this will produce a pointer to an uninitialized instance of Never. If this affects you, please read Porting.md!)

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test