Tweak closure string when using -Zspan_free_formats

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Implementation of (an interpretation of) the proposed output in https://internals.rust-lang.org/t/post-0/23184 for closures.

For closures in the local crate, we look at where the closure comes from in order to give them a locally reasonable name:

  - closures assigned to binding `let a`, `{a}`
  - closures passed directly as arguments in function call,
    `{function_name#parameter_name_or_position}`
  - closures passed directly as arguments in method call,
    `{Trait::method#parameter_name_or_position}`
  - closures assigned to statics or consts, the item's name
  - closures returned from functions, methods and closures, `{return}`

Do not include implicit desugared closure from async fn in closure type path: async_fn_name instead of async_fn_name::{closure#0}.