Improve the representation of operator expressions like "x + y" within

C++ templates. In particular, keep track of the overloaded operators
that are visible from the template definition, so that they can be
merged with those operators visible via argument-dependent lookup at
instantiation time. 

Refactored the lookup routines for argument-dependent lookup and for
operator name lookup, so they can be called without immediately adding
the results to an overload set.

Instantiation of these expressions is completely wrong. I'll work on
that next.

llvm-svn: 66851