[cxx-interop] Fix lookup of member operators
Calling `StructDecl::lookupDirect` with an operator identifier (e.g. `==`) previously returned no results. This happened because the underlying C++ operator function was added to the lookup table with an underscored name (e.g. `__operatorEqualEqual`), and the synthesized function was not added to the lookup table at all. Lookup should find the synthesized decl, since that is what Swift code will call.
This fixes a typechecker error when trying to conform a C++ struct that defines an operator to a Swift protocol with an operator requirement (e.g. `Equatable`).
