Dont strip const blocks in array lengths

[BoxyUwU]

r? oli-obk

mGCA now handles const blocks by always handling them during lower_expr_to_const_arg_direct instead of sometimes stripping them out at parse time. This is just generally a lot clearer/nicer but also means parsing isn't lossy which is just straight up wrong.

We now use MgcaDisambiguation::Direct for const blocks because we "directly" represent a const block as hir::ConstArgKind::Anon :> The only time that an anon const for const generics uses MgcaDisambiguation::AnonConst is for unbraced literals.

Once we properly support literals in hir::ConstArgKind (see #152139 #152001) then MgcaDisambiguation can be renamed to AnonConstKind with TypeSystem and NonTypeSystem variants. We can also get rid of mgca_direct_lit_hack. I expect this to be a very nice cleanup :)

Fixes rust-lang/rustfmt#6788

The diff relating to passing spans around is to avoid a bunch of mGCA diagnostics changing from const {} to {}. I'm not entirely sure why this was happening.

cc @rust-lang/rustfmt

How do I run the tests in the rustfmt repo from here? x test rustfmt only seems to run like 100 tests and doesn't result in a target/issue-6788.rs getting created. I've verified locally that this formats correctly though

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[BoxyUwU]

I don't think this should be beta backported to fix rustfmt so I'm going to file a separate PR for that

[oli-obk]

r=me with CI green