Reapply the commits to enable accurate hit-count detection for watchpoints.

This commit combines the initial commit (7c240de609af), a fix for x86_64 Linux
(3a0581501e76) and a fix for thinko in a last minute rewrite that I really
should have run the testsuite on.

Also, make sure that all the "I need to step over watchpoint" plans execute
before we call a public stop.  Otherwise, e.g. if you have N watchpoints and
a Signal, the signal stop info will get us to stop with the watchpoints in a
half-done state.

Differential Revision: https://reviews.llvm.org/D130674