Support glib-mkenums comment /*< flags >*/

Support glib-mkenums comment /*< flags >*/

- Modify the lexer to consider all "trigraph" comments specially, and
  parse them for "flags" as well as "private" and "public" (which were
  previously hardcoded).  This change allows for future support of
  multiple annotations inside a single trigraph comment.

- Change the parser to consider the additional field "flags" set by
  the lexer when constructing enums.

- Add a test case for the "flags" trigraph comment to the scanner
  annotation tests.

See <https://bugzilla.gnome.org/show_bug.cgi?id=631530>.