check-declare-locate is a compiled Lisp function in `check-declare.el'.

(check-declare-locate FILE BASEFILE)

Return the full path of FILE.
Expands files with a ".c" or ".m" extension relative to the Emacs
"src/" directory. Otherwise, `locate-library' searches for FILE.
If that fails, expands FILE relative to BASEFILE's directory part.
The returned file might not exist. If FILE has an "ext:" prefix, so does
the result.