[lldb] Make Process and subclass constructors protected

Make constructors of the Process and its subclasses class protected,
to prevent accidentally constructing Process on stack when it could be
afterwards accessed via a shared_ptr (since it uses
std::enable_shared_from_this<>).

The only place where a stack allocation was used were unittests,
and fixing them via declaring an explicit public constructor
in the respective mock classes is trivial.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D131275