Fix time-trace breaking flame graph assumptions

-ftime-trace could break flame-graph assumptions on Windows, with an
inner scope overrunning outer scopes. This was due to the way that times
were truncated. Changed this so time_points for the flame-graph are
truncated instead of durations, preserving the relative order of event
starts and ends.

I have tried to retain the extra precision for the totals, which count
thousands or millions of events.

Added assert to check this property holds in future.

Fixes PR43043

Differential Revision: https://reviews.llvm.org/D66411

llvm-svn: 371039