Add pointee: borrowing T to std.shared_ptr<T> for C++ interop

[mrousavy]

Motivation

Currently, std.shared_ptr<T>.pointee actually copies T, which is a big issue for pretty much every type wrapped in a std::shared_ptr that I have worked with - they're wrapped in a shared_ptr for a reason.

func doSomething(value: MySharedPtr) {
  value.pointee.doFirst()
  value.pointee.doSecond()
  value.pointee.doThird()
  // 3x copy, 3x destructor!
}

This is not only a performance concern, but maybe even a fundamental issue - some types are not expected to be copied and might delete resources unsafely in their destructors.

Proposed solution

It would be dope to add an API that allows us to get a borrowing T to avoid a copy.

Also, maybe this even allows for using non-copyable C++ types in Swift?

Alternatives considered

I can add SWIFT_IMMORTAL_REFERENCE to every type that I plan on using inside of a std::shared_ptr, but this isn't ideal either, as sometimes I use external types that are not written by me.

Additional information

No response