Refactored lldb executable name discovery

The lldb executable was referenced through the code by 7 different
(effectively) global variables.

global lldbExecutablePath
global lldbExecutable
os.environ['LLDB_EXEC']
os.environ['LLDB_TEST']
dotest.lldbExec
dotest.lldbHere
lldbtest.lldbExec

This change uses one global variable lldbtest_config.lldbExec to
replace them all.

Differential Revision: http://reviews.llvm.org/D9817

llvm-svn: 237600