Since this is a C++ stdlib type we need make sure that any overloads
that use it are preferred over custom types that also conform to
`ExpressibleByStringLiteral` when argument is a string literal.
This is important for operators like `==` which could be heterogenous
and have a custom C++ type that conforms to `ExpressibleByStringLiteral`
on either side together with `std.string` i.e.
`==(std.string, const CustomString &)`, such overloads should only
be selected if argument passed to `CustomString` is non-literal because
literals are convered by a stdlib `==(std.string, std.string)` overload.
If the C++ type of a function parameter defines a custom copy constructor, assume that it is safe to use from Swift. This matches the heuristic that we use to detect if a C++ method is safe based on the return type.
rdar://121391798
