Add support for ./dotest.py --channel and --log-success

Summary:
New dotest options that allow arbitrary log channels and
categories to be enabled.  Also enables logging for locally run
debug servers.
    
Log messages are separated into separate files per test case.
(this makes it possible to log in dosep runs)
    
These new log files are stored side-by-side with trace files in the
session directory.
    
These files are deleted by default if the test run is successful.
    
If --log-success is specified, even successful logs are retained.
    
--log-success is useful for creating reference log files.
    
Test Plan:
add '--channel "lldb all" --channel "gdb-remote packets" --log-success'
to your dotest options

Tested on OSX and Linux
    
Differential Revision: http://reviews.llvm.org/D9594

llvm-svn: 236956