Add a version of std::function that includes a few optimizations in ABI V2.

Patch by Jordan Soyke (jsoyke@google.com)
Reviewed as D55045

The result of running the benchmarks and comparing them can be found
here: https://gist.github.com/EricWF/a77fd42ec87fc98da8039e26d0349498

llvm-svn: 348812