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win: Move Parallel.h off concrt to cross-platform code

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win: Move Parallel.h off concrt to cross-platform code

r179397 added Parallel.h and implemented it terms of concrt in 2013.

In 2015, a cross-platform implementation of the functions has appeared
and is in use everywhere but on Windows (r232419). r246219 hints that
<thread> had issues in MSVC2013, but r296906 suggests they've been fixed
now that we require 2015+.

So remove the concrt code. It's less code, and it sounds like concrt has
conceptual and performance issues, see PR41198.

I built blink_core.dll in a debug component build with full symbols and
in a release component build without any symbols. I couldn't measure a
performance difference for linking blink_core.dll before and after this
patch.

Differential Revision: https://reviews.llvm.org/D68820

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