[lldb][NFC] Cleanup ValueObject construction code

Just code cleanup for ValueObject constructors:

* Use default member initializers where possible.
* Doxygenify the comments for membersa nd constructors where needed.
* Delete the default constructor which isn't defined.
* Initialize the bitfields via a utility struct instead of doing this in the
  different constructors.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D97199