Make ArgumentsAdjuster an std::function.

Reviewers: klimek

Reviewed By: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6505
Make ArgumentsAdjuster an std::function (clang-tools-extra part of D6505).

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6506