[OrderedDictionary] Resolve call-site ambiguities

[lorentey]

Add @_disfavoredOverload attributes to resolve edge-case ambiguities with overloads that only differ between tuple labels. In some situations, the compiler seems to be of two minds about whether (Key, Value) is compatible with (key: Key, value: Value) or vice versa.

@_disfavoredOverload is not a stable attribute; using it involves some risk in case future compilers remove it or change its behavior. In the worst case, we may have to publish a new release to fix such issues.

Resolves #125.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[glessard]

Nice!