Add the real benchmarks comparing lldb against gdb for repeated expression evaluations.

Modify lldbbench.py so that lldbtest.line_number() utility function is available to
BenchBase client as just line_number(), and modify lldbtest.py so that self.lldbExec
(the full path for the 'lldb' executable) is available to BenchBase client as well.

An example run of the test case on my MacBook Pro running Lion:

1: test_compare_lldb_to_gdb (TestRepeatedExprs.RepeatedExprsCase)
   Test repeated expressions with lldb vs. gdb. ... 
lldb_avg: 0.204339
gdb_avg: 0.205721
lldb_avg/gdb_avg: 0.993284
ok

llvm-svn: 136740