[NewPM] Add support for new-PM plugins to clang

Summary:
This adds support for new-PM plugin loading to clang. The option
`-fpass-plugin=` may be used to specify a dynamic shared object file
that adheres to the PassPlugin API.

Tested: created simple plugin that registers an EP callback; with optimization level > 0, the pass is run as expected.

Committed on behalf of Marco Elver

Differential Revision: https://reviews.llvm.org/D56935

llvm-svn: 352972