Reduce number of threads in lldb-mi.

LLDB-mi have 3 threads.

1. Wait for input.
2. Process commands.
3. Process events.
This revision merges 1 & 2. Same thread waits on input and then process the
command. This way, no synchronization is needed between first and 2nd. Also it is
easy to check when to exit.

A lot of code will redundant and will be cleaned up gradually.

All lldb-mi tests pass with gcc and clang as test compiler. Also did minimal testing
on command line and works ok. The "quit" and "-gdb-exit" command close the application
without needing any further return.

Reviewed in http://reviews.llvm.org/D7746.

llvm-svn: 230003