[Support] Simplify parallelForEach{,N}

* Merge parallel_for_each into parallelForEach (this removes 1 `Fn(...)` call)
* Change parallelForEach to use parallelForEachN
* Move parallelForEachN into Parallel.cpp

My x86-64 `lld` executable is 100KiB smaller.
No noticeable difference in performance.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D117510