[Dexter] Add timeout options

Adds a pair of options for Dexter that allow the user to specify a
timeout duration. These options are:

* --timeout-total: Times out if the total run-time of the debugger session
  exceeds <timeout-total> seconds.
* --timeout-breakpoint: Times out if the time without hitting a
  breakpoint exceeds <timeout-breakpoint> seconds.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D145063