[lldb][NFC] Move various constructor definitions from .h to .cpp

I ran into issues with linking downstream language plugin to liblldb in
debug builds, hitting link time errors of form:

```
undefined reference to `vtable for  lldb_private::<Insert LLDB class here>'
```

Anchoring the vtable to the .cpp files resolved those issues.

Differential Revision: https://reviews.llvm.org/D147252