Support formatv of TimePoint with strftime-style formats.

Summary:
Support formatv of TimePoint with strftime-style formats.

Extensions for millis/micros/nanos are added.
Inital use case is HH:MM:SS.MMM timestamps in clangd logs.

Reviewers: bkramer, ilya-biryukov

Subscribers: labath, llvm-commits

Differential Revision: https://reviews.llvm.org/D38992

llvm-svn: 316419