Add a new attribute self.lldbHere, representing the fullpath to the 'lldb' executable

built locally from the source tree.  This is distinguished from self.lldbExec, which
can be used by test/benchmarks to measure the performances against other debuggers.

You can use environment variable LLDB_EXEC to specify self.lldbExec to the dotest.py
test driver, otherwise it is going to be populated with self.lldbHere.

Modify the regular tests under test dir, i.e., not test/benchmarks, to use self.lldbHere.
Also modify the benchmarks tests to use self.lldbHere when it needs an 'lldb' executable
with debug info to do the performance measurements.

llvm-svn: 138608