Add a benchmark comparing lldb vs. gdb with disassembly on a large function...

Add a benchmark comparing lldb vs. gdb with disassembly on a large function (lldb's Driver::MainLoop()).

Sample run on my OSX Lion (MacBook Pro):

1: test_run_gdb_then_lldb (TestDisassembly.DisassembleDriverMainLoop)
   Test disassembly on a large function with lldb vs. gdb. ... 
gdb benchmark: Avg: 0.201802 (Laps: 5, Total Elapsed Time: 1.009008)
lldb benchmark: Avg: 0.109569 (Laps: 5, Total Elapsed Time: 0.547843)
lldb_avg/gdb_avg: 0.542952
ok
2: test_run_lldb_then_gdb (TestDisassembly.DisassembleDriverMainLoop)
   Test disassembly on a large function with lldb vs. gdb. ... 
lldb benchmark: Avg: 0.109580 (Laps: 5, Total Elapsed Time: 0.547902)
gdb benchmark: Avg: 0.201587 (Laps: 5, Total Elapsed Time: 1.007936)
lldb_avg/gdb_avg: 0.543588
ok

llvm-svn: 136931