Add a benchmark for measuring the response time of the 'frame variable' command.

Example (start the lldb inferior, break at the Driver::MainLoop() function, and
issue 'frame variable'):

 $ ./dotest.py -v +b -x '-F Driver::MainLoop()' -n -p TestFrameVariableResponse.py
----------------------------------------------------------------------
Collected 1 test

1: test_startup_delay (TestFrameVariableResponse.FrameVariableResponseBench)
   Test response time for the 'frame variable' command. ... 
lldb frame variable benchmark: Avg: 1.636897 (Laps: 20, Total Elapsed Time: 32.737944)
ok

----------------------------------------------------------------------
Ran 1 test in 65.105s

OK

llvm-svn: 142678