Add a benchmark for measuring start up delays of lldb, including:

o create a fresh target; and
o set the first breakpoint

Example (using lldb to set a breakpoint on lldb's Driver::MainLoop function):

./dotest.py -v +b -x '-F Driver::MainLoop()' -p TestStartupDelays.py

...

1: test_startup_delay (TestStartupDelays.StartupDelaysBench)
   Test start up delays creating a target and setting a breakpoint. ... 
lldb startup delays benchmark:
create fresh target: Avg: 0.106732 (Laps: 15, Total Elapsed Time: 1.600985)
set first breakpoint: Avg: 0.102589 (Laps: 15, Total Elapsed Time: 1.538832)
ok

llvm-svn: 142628