syntax: improve parameter without type suggestions
Fixes#64252.
This PR improves the suggestions provided when function parameters
do not have types:
- A new suggestion is added for arbitrary self types, which suggests
adding `self: ` before the type.
- Existing suggestions are now provided when a `<` is found where a `:`
was expected (previously only `,` and `)` or trait items), this gives
suggestions in the case where the unnamed parameter type is generic
in a free function.
- The suggestion that a type name be provided (e.g. `fn foo(HashMap<u32>)`
-> `fn foo(HashMap: TypeName<u32>)`) will no longer occur when a `<` was
found instead of `:`.
- The ident will not be used for recovery when a `<` was found instead
of `:`.
r? @Centril
cc @estebank @yoshuawuyts
faf2e8087c