[ADT] Add a default constructor and a bool conversion to function_ref.

[ADT] Add a default constructor and a bool conversion to function_ref.

The internal representation has a natural way to handle this and it
seems nicer than having to wrap this in an optional (with its own
separate flag).

This also matches how std::function works.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307490 91177308-0d34-0410-b5e6-96231b3b80d8