[dexter] Add hit_count keyword arg to DexLimitSteps

The DexLimitSteps command leading breakpoint will be deleted after triggering
'hit_count' number of times if the argument is provided.

All the lit tests pass on linux (with lldb), and I've tested this on windows
(with vs2017) manually as the lit tests for DexLimitSteps are currently
unsupported on windows.

Reviewed By: StephenTozer, chrisjackson

Differential Revision: https://reviews.llvm.org/D101527