[Driver] Add callback to Command execution

Summary:
Object of type `Compilation` now can keep a callback that is called
after each execution of `Command`. This must simplify adaptation of
clang in custom distributions and allow facilities like collection of
execution statistics.

Reviewers: rsmith, rjmccall, Eugene.Zelenko

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78899