Warning for struct copy of selected types

[gafter]

See item 1 from dotnet/csharplang#1502.

Please add a way to get warnings for situations in which the compiler would or does produce a defensive copy for a struct type that has some particular new attribute on it. This should be under control of a warning wave (i.e. the warning is off by default so as to reduce compatibility issues*).

*the compatibility concern is that code that uses the attribute and for which there is a defensive copy produces no diagnostic in an older version of the compiler, but does produce a diagnostic in a new version of the compiler.

[sharwell]

🎉 This would likely help with the work in #26321. Conceptually, this is an attempt to implement a lightweight std::unique_ptr<T>, but it's difficult without having an equivalent to boost::noncopyable.

[sharwell]

For me, important semantics here include:

• Non-copyable is unrelated to pinned/fixed (a pinned value may or may not be copied, and a non-copyable value may or may not be pinned).
• If a non-copyable value is moved from one location in memory to another location in memory, the source location is treated as not-definitely-assigned.
• Boxing a value type counts as moving the value from its location on the stack to another location on the heap.
• Once boxed, a value cannot be moved back to the stack (unless it's known that there are no other references to the box, optionally). The value is still usable by reference using Unsafe.Unbox<T>.
• A reference to a non-copyable value, including the this reference appearing in instance methods of the type, cannot be assumed to reference a non-boxed value. As such, this value cannot be moved.