Revert the fix for PR8013.

That bug concerned the well-formedness of code such as (&ovl)(a, b,
c). GCC rejects the code, while EDG accepts it. On further study of the
standard, I see no support for EDG's position: in particular, C++
[over.over] does not list this as a context where we can take the
address of an overloaded function, C++ [over.call.func] does not
reference the address-of operator at any point, and C++ [expr.call]
claims that the function argument in a call is either a function
lvalue or a pointer-to-function; (&ovl) is neither.

llvm-svn: 118620