Add `fn fn_decl` to `Hir`, for looking up the `FnDecl` of a body owner.
Add `fn fn_decl` to `Hir`, for looking up the `FnDecl` of a body owner.

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