Add element atomic memset intrinsic

Summary: Continuing the work from https://reviews.llvm.org/D33240, this change introduces an element unordered-atomic memset intrinsic. This intrinsic is essentially memset with the implementation requirement that all stores used for the assignment are done with unordered-atomic stores of a given element size.

Reviewers: eli.friedman, reames, mkazantsev, skatkov

Reviewed By: reames

Subscribers: jfb, dschuff, sbc100, jgravelle-google, aheejin, efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D34885

llvm-svn: 307854