lint: fix bug when parsing unused variable (since 2021-07-10)

Partially revert to cgram.y 1.248 from 2021-06-29.

This fixes the parse error for variables whose declaration starts with
__attribute__((unused)).  In the many refactorings of the last days this
bug has slipped in, and since there were several refactorings in that
area, there may be have been further bugs that are not caught by the
current test suite.  Revert for now and maybe apply them later again
when there are more tests.

Things kept from the current version are:

The names of most of the rules, as they correspond more closely to C99
and do not affect the behavior in any way.

In type_direct_decl, the replacement of type_attribute_list with
type_attribute since that nonterminal is already part of a repetition
(saves 4 conflicts).

In block_item, the order of the rules corresponds to C99.  This has no
influence on the generated parser, except for the rule numbers, which
are informative.

The merge of the duplicate code for struct_tag, enum_tag and
enum_constant, as they all contained exactly the same code.

(rillig)