Make File option flags consistent for Python API

Summary:
Fixes SBCommandReturnObject::SetImmediateOutputFile() and
SBCommandReturnObject::SetImmediateOutputFile() for files opened
with "a" or "a+" by resolving inconsistencies between File and
our Python parsing of file objects.

Reviewers: granata.enrico, Eugene.Zelenko, jingham, clayborg

Subscribers: lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D18228

Change by Francis Ricci <fjricci@fb.com>

This is a cherry-pick of r264351

llvm-svn: 270123