Support for truncate/append on log files

Summary:
Presently, if a log file already exists, lldb simply starts overwriting bits of it, without
truncating or anything. This patch makes it use eFileOptionFileTruncate by default. It also adds
an --append option, which will append to the file without truncating. A test is included.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8450

llvm-svn: 232801