Recover param: Ty = EXPR

[compiler-errors]

Fixes #137310

Pretty basic recovery here, but better than giving an unexpected token error.

[rustbot]

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[BoxyUwU]

r? fmease

[fmease]

On a second glance, I would think parse_param_general is 'a better fit' for this compared to parse_ty_for_param (which should probably only concern itself with parsing types).

Mostly an organizational nitpick since it shouldn't be able to affect behavior (CC anon params in Rust 2015). I was only able to find trait T { fn f((x)=0 fn) {} } which will emit two errors under this PR in Rust 2015 (parameter defaults are not supported and expected `:`, found `=`) instead of one and would keep emitting a single one if the logic was moved into parse_param_general I think but that's hardly interesting.

nw, r=me then

r=me with or without moving the logic into parse_param_general. Sorry for the churn / wrong signal there.

[fmease]

(preexisting: the patterns aren't allowed in methods without bodies parser recovery is fragile already: it suggests the more butchered _: )=0 for trait T { fn f((x)=0); } under Rust 2015)