mir-opt tests: add ability to match lines that contain the filename

[RalfJung]

As part of #43403, I wanted to write mir-opt tests that check lines that contain types which involve the name of the file was was compiled (this is because with closures, that filename leaks into the name of the type as printed). It is currently not possible to write such a test as the filename is not stable.

So, this PR introduces the magic variable $FILE that can be used in mir-opt tests to refer to the name of the current file, so that it can be used to test against.

[rust-highfive]

r? @aturon

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[RalfJung]

Cc @nikomatsakis

[aturon]

@eddyb @arielb1, one of you up for reviewing this?

[arielb1]

rustc has the -Z span_free_formats option that disables span emission for closures, which is used say in https://github.com/rust-lang/rust/blob/master/src/test/mir-opt/end_region_7.rs#L11. Does that work for you @RalfJung ?

[RalfJung]

Yes, that looks great, thanks!