Preserve escape sequences in documentation comments on macro expansion

Preserve escape sequences in documentation comments on macro expansion

Escape sequences in documentation comments must not be parsed as a
normal string when expanding a macro, otherwise some innocent but
invalid-escape-sequence-looking comments will trigger an ICE.

Although this commit replaces normal string literals with raw string
literals in macro expansion, this shouldn't be much a problem
considering documentation comments are converted into attributes before
being passed to a macro anyways.

Fixes #25929.
Fixes #25943.