This is step 2 towards fixing #77548.
In the codegen and codegen-units test suites, the `//` comment markers
were kept in order not to affect any source locations. This is because
these tests cannot be automatically `--bless`ed.
- src links/redirects to extern fn from another crate had an extra '/'.
- src links to `pub use` of a crate module had an extra '/'.
- src links to renamed reexports from another crate used the new name
for the link but should use the original name.
