libcpp: move label_text to its own header

No functional change intended.

libcpp/ChangeLog:
	* Makefile.in (TAGS_SOURCES): Add include/label-text.h.
	* include/label-text.h: New file.
	* include/rich-location.h: Include "label-text.h".
	(class label_text): Move to label-text.h.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>