[Dexter] Add on_line parameter to DexExpectStepOrder command

DexExpectStepOrder uses the line to expect a debugger step from the actual line
of the command in the Dexter source file. Now Dexter scripts have mainly moved
to thier own script files instead of the actual source, there should be a
option to override this behaviour to choose your own debugger step location.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D142099