This is a copy of rust-lang#4296 with these changes:
* file is not reopened again to find if the file is generated
* first five lines are scanned for `@generated` marker instead of one
* no attempt is made to only search for marker in comments

`@generated` marker is used by certain tools to understand that the
file is generated, so it should be treated differently than a file
written by a human:
* linters should not be invoked on these files,
* diffs in these files are less important,
* and these files should not be reformatted.

This PR proposes builtin support for `@generated` marker.

I have not found a standard for a generated file marker, but:
* Facebook [uses `@generated` marker](https://tinyurl.com/fb-generated)
* Phabricator tool which was spawned from Facebook internal tool
  [also understands `@generated` marker](https://git.io/JnVHa)
* Cargo inserts `@generated` marker into [generated Cargo.lock files](https://git.io/JnVHP)

My personal story is that rust-protobuf project which I maintain
was broken twice because of incompatibilities/bugs in rustfmt marker
handling: [one](stepancheg/rust-protobuf#493),
[two](stepancheg/rust-protobuf#551).
(Also, rust-protobuf started generating `@generated` marker
[6 years ago](https://git.io/JnV5h)).

While rustfmt AST markers are useful to apply to a certain AST
elements, disable whole-file-at-once all-tools-at-once text level
marker might be easier to use and more reliable for generated code.