[cxx-interop] Out-of-line overloaded operators are printed twice in the module interface

[egorzhdan]

See test/Interop/Cxx/namespace/Inputs/free-functions.h:

namespace FunctionsNS1 {
struct X {};
inline const char *operator+(X, X) { return "FunctionsNS1::operator+(X, X)"; }
}

In the generated module interface, operator+ is printed twice: once as a member of enum FunctionsNS1, and once again as a global function. The operator is actually imported as a global function, and Swift should only print it as a global function.

(test/Interop/Cxx/namespace/free-functions-module-interface.swift)