[Pitch] Introduce a way to annotate functions that are called at most once

Hello Swift community! This pitch introduces a @called(once) attribute to model at-most once execution of a function. Making function execution behavior visible to the compiler has two advantages. First, it makes it possible to diagnose incorrect usage of such functions. But, it also allows the compiler to understand how closure captures could affect variable lifetimes. This can improve compatibility with non-Copyable types. A complete proposal could be found here - Proposal: `@called(once)` a...