lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

lint: remove compat code for lexing hex floating point literals

Hex floating literals are required by C99.

tests/lint: expect complete messages in feature tests

Previously, the tests contained many comments like /* expect: 123 */,
which were useless to a casual reader since nobody is expected to learn
lint's message IDs by heart. Replace these with the complete
diagnostics, to show what lint is complaining about.

The tests named msg_*.c have been left unmodified since they mention the
full message text in their header comment.

No functional change.

tests/lint: add test cases for lexical analysis