[Support] Add FormatVariadic support for chrono types

Summary:
The formatter has three knobs:
- the user can choose which time unit to use for formatting (default: whatever is the unit of the input)
- he can choose whether the unit gets displayed (default: yes)
- he can affect the way the number itself is formatted via standard number formatting options (default:default)

Reviewers: zturner, inglorion

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29481

llvm-svn: 294326