lint: merge duplicate code for struct and enum tags

This is a tricky area.  Inlining identifier_sym did not work because of
the side-effects of getsym.  In situations where two identifiers of
different kinds follow each other, such as 'enum id1 id2', the timing is
crucial, and inlining identifier_sym would have led to an internal error
in getsym.  The symbol type has to be reset from FTAG to FVFT before
reading the next token.

No functional change.

(rillig)