[llvm] [SystemZ][z/OS] Add UtcClock extension to chrono.h/.cpp (PR #67846)

[Yusra Syeda via llvm-commits]

ysyeda wrote:

> Are there patches outstanding that'll use this in the LLVM project?

@dwblaikie This patch is required for a change in the SystemZ backend for z/OS: https://github.com/llvm/llvm-project/pull/65407 I will post the PR once this one has been merged. Thanks for your review.

https://github.com/llvm/llvm-project/pull/67846