Support for calling overloaded function call operators (operator())

with function call syntax, e.g.,

  Functor f;
  f(x, y);

This is the easy part of handling calls to objects of class type 
(C++ [over.call.object]). The hard part (coping with conversions from
f to function pointer or reference types) will come later. Nobody uses
that stuff anyway, right? :)

llvm-svn: 59663