[PassManager] Make PassManagerBuilder::addExtension take an std::function, rather...

[PassManager] Make PassManagerBuilder::addExtension take an std::function, rather than a function pointer.

Summary:
This gives callers flexibility to pass lambdas with captures, which lets
callers avoid the C-style void*-ptr closure style.  (Currently, callers
in clang store state in the PassManagerBuilderBase arg.)

No functional change, and the new API is backwards-compatible.

Reviewers: chandlerc

Subscribers: joker.eph, cfe-commits

Differential Revision: http://reviews.llvm.org/D18613

llvm-svn: 264918