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/normalize.rs

@@ -1,4 +1,5 @@
 //! Deeply normalize types using the old trait solver.
+
 use super::error_reporting::OverflowCause;
 use super::error_reporting::TypeErrCtxtExt;
 use super::SelectionContext;