[Core] Add parallel_for_each
[Core] Add parallel_for_each

This adds a parallel_for_each similar to functionality in MSVC concurrency
library.

This was very patiently reviewed by Rui and credits go to him for this patch.

Differential Revision: http://reviews.llvm.org/D8348

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@232419 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld