Add blank lines after module-level //! comments.

Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -6,6 +6,7 @@
 //!
 //! [rustc dev guide]:
 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
+
 use rustc_ast::Mutability;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;