Avoid std::function until PR19030 is fixed

We'd like to keep the clang-cl self-host working until we implement
MSVC-compatible RTTI.

Differential Revision: http://llvm-reviews.chandlerc.com/D2930

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202758 91177308-0d34-0410-b5e6-96231b3b80d8