Rollup merge of #138284 - compiler-errors:const-param-ty-annotation, r=BoxyUwU

Do not write user type annotation for const param value path

As I noted in the code comment, `DefKind::ConstParam` isn't actually *generic* over its own args, we just use the identity args from the body when lowering the value path so we have something to plug into the `EarlyBinder` we get back from `type_of` for the const param. So skip over it in `write_user_type_annotation_from_args`.

Somewhat unrelated, but I left an explanation for a somewhat mysterious quirk in the THIR lowering of user type annotations for patterns having to do with ctors and their `type_of` not actually being the type of the pattern node it's ascribing.

Fixes #138048

r? ``@BoxyUwU``

Author: matthiaskrgr