This fixes a build warning:
```
swift/stdlib/public/Cxx/CxxDictionary.swift:168:9: warning: variable 'iter' was never mutated; consider changing to 'let' constant
```
This adds an automatic conformance for `std::map` and `std::unordered_map` to Swift's `ExpressibleByDictionaryLiteral` protocol.
This makes it possible to pass a dictionary literal as an argument to a function that takes a `std::map` as parameter.
rdar://137126474
This adds a protocol to the C++ standard library overlay which will improve the ergonomics of `std::map` and `std::unordered_map` when used from Swift code.
As of now, `CxxDictionary` adds a subscript with an optional return type that mimics the subscript of `Swift.Dictionary`.
Similar to https://github.com/apple/swift/pull/63244.
