fix D1422343 ("make `folly::Formatter` extendible") for clang

fix D1422343 ("make `folly::Formatter` extendible") for clang

Summary:
This is D1422343, but with a one-line change to make it clang-compatible. I authored this diff by first copying the original, then updating, so it's easy to see what I had to change.

> on advice of @tudorb, move most of `folly::Formatter` into `folly::BaseFormatter` so that we can use compile-time polymorphism to provide different types of Formatters.

Test Plan:
```
fbmake clean
fbconfig -r thrift folly cold_storage && fbmake dbg && fbmake runtests
fbmake clean
fbconfig -r hphp --clang && fbmake dbgo && fbmake runtests
```

Reviewed By: tudorb@fb.com, pt@fb.com

Subscribers: mathieubaudet, tudorb

FB internal diff: D1440310

Tasks: 4624268, 4667712