[support] Prepare TimeProfiler for cross-thread support

This NFC prepares the TimeProfiler to support the construction
and completion of time profiling 'entries' across threads.

Add ClockType alias so we can change the clock in one place.
(trivial) Use c++ usings instead of typedefs
Rename Entry to TimeTraceProfilerEntry since this type will eventually become public.
Add an intro comment.
Add some smoke unit tests.

Reviewed By: russell.gallop, rriddle, lattner, jloser

Differential Revision: https://reviews.llvm.org/D133153