refactor: move IOObject::m_should_close_fd into subclasses

Summary:
m_should_close_fd doesn't need to be in IOObject.   It will be useful
for my next change to move it down into File and Socket.

Reviewers: labath, JDevlieghere, jasonmolenda

Reviewed By: JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68152

llvm-svn: 373126