lint: remove noclass_declmods from grammar

That rule was hard to understand since it contained clrtyp but not the
corresponding deftyp. It's easier to read when a grammar rule contains
both clrtyp and deftyp in the same line.

C99 does not mention the term 'decl-modifier' anywhere, and in fact the
only thing that this grammar rule allowed was a type-qualifier-list.
The new name better reflect this.

No functional change.

lint: add common header for all tests

For those tests that didn't use GCC-style line markers such as "# 2",
the line numbers of the diagnostics stay the same. This is purely
conincidental. Before, the 3 lines came from lint's built-in
definitions (see 'builtins' in main1.c), and line number counting
continued as if nothing had happened, making the first line of the
actual file line 4. These 3 built-in lines are now replaced with 3
lines of file header.

lint: normalize whitespace in tests

Add an anonymous struct test