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_session/src/output.rs

@@ -1,4 +1,5 @@
 //! Related to out filenames of compilation (e.g. binaries).
+
 use crate::config::{self, CrateType, Input, OutFileName, OutputFilenames, OutputType};
 use crate::errors::{
     self, CrateNameDoesNotMatch, CrateNameEmpty, CrateNameInvalid, FileIsNotWriteable,