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[lit] Add --time-trace-output to lit

Authored by russell.gallop on Aug 4 2020, 2:14 AM.

Description

[lit] Add --time-trace-output to lit

This produces a chrome://tracing compatible trace file in the same way
as -ftime-trace.

This can be useful in optimising test time where one long test is causing
long overall test time on a wide machine.

This also helped in finding tests which have side effects on others
(e.g. https://reviews.llvm.org/D84885).

Differential Revision: https://reviews.llvm.org/D84931