The breakpoint location hit counts were getting incremented in
BreakpointLocation::ShouldStop.  That worked but wasn't really right,
since there's nothing to guarantee that won't get called more than
once.  So this change moves that responsibility to the StopInfoBreakpoint
directly, and then it uses the BreakpointSite to actually do the bumping.

Also fix a test case that was assuming if you had many threads running some
code with a breakpoint in it, the hit count when you stopped would always be
1.  Many of the threads could have hit it at the same time...

<rdar://problem/18577603>