Match w32 paths in grep sans --null hits (Bug#32051)

Match w32 paths in grep sans --null hits (Bug#32051)

* lisp/progmodes/grep.el (grep-regexp-alist): Add an optional part to
match paths starting with C: (other drive letters).
* test/lisp/progmodes/compile-tests.el
(compile-tests--grep-regexp-testcases)
(compile-tests--grep-regexp-tricky-testcases)
(compile-test-grep-regexps): New tests.
(compile--test-error-line): Return `compilation-message'.