Thread more argument info down from `Hir` into the `mir::LocalDecls`.
Thread more argument info down from `Hir` into the `mir::LocalDecls`.

Namely, we thread down the `HirId` of the explicit type of the
argument.  In the case of the special `self` variable with an implicit
type, we also thread down a description of its structure (`self`/`mut
self`/`&self`/`&mut self`).

GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust