Clean and format `#include` directives.

Change-Id: Iba5a0f3a18245a22f68ba1c3d533200cc577a596
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63130
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
60 files changed