Refactor the way we handle operator overloading and template

instantiation for binary operators. This change moves most of the
operator-overloading code from the parser action ActOnBinOp to a new,
parser-independent semantic checking routine CreateOverloadedBinOp. 

Of particular importance is the fact that CreateOverloadedBinOp does
*not* perform any name lookup based on the current parsing context (it
doesn't take a Scope*), since it has to be usable during template
instantiation, when there is no scope information. Rather, it takes a
pre-computed set of functions that are visible from the context or via
argument-dependent lookup, and adds to that set any member operators
and built-in operator candidates. The set of functions is computed in
the parser action ActOnBinOp based on the current context (both
operator name lookup and argument-dependent lookup). Within a
template, the set computed by ActOnBinOp is saved within the
type-dependent AST node and is augmented with the results of
argument-dependent name lookup at instantiation time (see
TemplateExprInstantiator::VisitCXXOperatorCallExpr).

Sadly, we can't fully test this yet. I'll follow up with template
instantiation for sizeof so that the real fun can begin.

llvm-svn: 66923