[Driver] Refactor distro detection & classification as a separate API

Refactor the Distro enum along with helper functions into a full-fledged
Distro class, inspired by llvm::Triple, and make it a public API.
The new class wraps the enum with necessary comparison operators, adding
the convenience Is*() methods and a constructor performing
the detection. The public API is needed to run the unit tests (D25869).

Differential Revision: https://reviews.llvm.org/D25949

llvm-svn: 288060