Keep and report "foreign" diangnostics in flymake-cc Flymake backend

Keep and report "foreign" diangnostics in flymake-cc Flymake backend

This includes diagnostics for .h files that sprang up when checking a
c file.  Those diagnostics are reported to the Flymake infrastructure
which does not (yet) do anything with them.

This includes a change to the test fixtures, too.

* lisp/progmodes/flymake-cc.el (flymake-cc--make-diagnostics): Rework

* test/lisp/progmodes/flymake-resources/another-problematic-file.c:
  New file.

* test/lisp/progmodes/flymake-resources/some-problems.h:
  Add a function declaration..