Generalize the pass registration mechanism used by Polly to any third-party tool

There's quite a lot of references to Polly in the LLVM CMake codebase. However
the registration pattern used by Polly could be useful to other external
projects: thanks to that mechanism it would be possible to develop LLVM
extension without touching the LLVM code base.

This patch has two effects:

1. Remove all code specific to Polly in the llvm/clang codebase, replaicing it
   with a generic mechanism

2. Provide a generic mechanism to register compiler extensions.

A compiler extension is similar to a pass plugin, with the notable difference
that the compiler extension can be configured to be built dynamically (like
plugins) or statically (like regular passes).

As a result, people willing to add extra passes to clang/opt can do it using a
separate code repo, but still have their pass be linked in clang/opt as built-in
passes.

Differential Revision: https://reviews.llvm.org/D61446