Add a new compile error regexp for Clang includes

Add a new compile error regexp for Clang includes

Clang uses a slight variation of GCC's include format, causing includes
to be treated as warnings instead of informational messages.  Use a new
regular expression instead.

* lisp/progmodes/compile.el
(compilation-error-regexp-alist-alist): New element
`clang-include' for Clang-style "included from" lines.
* test/lisp/progmodes/compile-tests.el
(compile-tests--test-regexps-data): Add unit test.