Add a Stopwatch utility class to lldbench.py module and initialize an instance of

Stopwatch (self.swatch) within the BenchBase's setUp() instance method to be available
to all the child classes.

Use self.swatch to measure elapsed time in TestRepeatedExprs.py, which needs to be
modified later on to actually measure repeated expression evaluations within the
context of lldb as well as gdb.

llvm-svn: 136664