Bring back beginning_vert field to ast::Arm

[topecongiro]

beginning_vert field was removed from ast::Arm in the stabilization PR, but rustfmt needs this to properly format a match arm with | at the beginning, so please bring this back!

[rust-highfive]

r? @eddyb

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[goodmanjonathan]

Heh, sorry about that!

[petrochenkov]

@topecongiro
How rustfmt deals with trailing commas and other separators? We don't keep them in AST.
The starting | is exactly equivalent to them.

[petrochenkov]

I'd rather not add this | into AST without treating other trailing separators consistently.
IIRC, you can check for the presence of | by searching in the codemap, even if it's not in AST.

[pietroalbini]

@eddyb (or someone else from @rust-lang/compiler), can we get a review on this PR?

[petrochenkov]

There's a review above.

[pietroalbini]

Oh, woops!

[petrochenkov]

I'll go ahead and close then.

[eddyb]

r? @petrochenkov

[eddyb]

Sorry, GitHub didn't update.