Remove the recommendation against using std::function

Clang-cl supports MSVC-style RTTI now, and we can even compile
typeid(...) with /GR-.  Just don't instantiate std::function with a
polymorphic type, or bad things will happen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212148 91177308-0d34-0410-b5e6-96231b3b80d8