Add bench.py as a driver script to run some benchmarks on lldb.

Add benchmarks for expression evaluations (TestExpressionCmd.py) and disassembly (TestDoAttachThenDisassembly.py).

An example:
[17:45:55] johnny:/Volumes/data/lldb/svn/trunk/test $ ./bench.py 2>&1 | grep -P '^lldb.*benchmark:'
lldb startup delay (create fresh target) benchmark: Avg: 0.104274 (Laps: 30, Total Elapsed Time: 3.128214)
lldb startup delay (set first breakpoint) benchmark: Avg: 0.102216 (Laps: 30, Total Elapsed Time: 3.066470)
lldb frame variable benchmark: Avg: 1.649162 (Laps: 20, Total Elapsed Time: 32.983245)
lldb stepping benchmark: Avg: 0.104409 (Laps: 50, Total Elapsed Time: 5.220461)
lldb expr cmd benchmark: Avg: 0.206774 (Laps: 25, Total Elapsed Time: 5.169350)
lldb disassembly benchmark: Avg: 0.089086 (Laps: 10, Total Elapsed Time: 0.890859)

llvm-svn: 142708