Refactor std::function to more easily support alternative implementations.

Patch from Jordan Soyke (jsoyke@google.com)
Reviewed as D55520

This change adds a new internal class, called __value_func, that adds
a minimal subset of value-type semantics to the internal __func interface.

The change is NFC, and is cleanup for the upcoming ABI v2 function implementation (D55045).

llvm-svn: 348778