SBFile support in SBCommandReturnObject

Summary:
This patch add SBFile interfaces to SBCommandReturnObject, and
removes the internal callers of its FILE* interfaces.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68434

llvm-svn: 374238