Just using `check` as a prefix to various keywords would lead to too many
ambiguities in the parser (notably in contracts, where you can have an empty
requires list followed by `check ensures ...` or a `check requires ...`. With
a lookahead of 1, it's difficult for the parser see which rule should apply.
Hence, the lexer will perform itself the lookahead and emit a proper token
(`CHECK_ENSURES` or `CHECK_REQUIRES` respectively).

a _noreturn attribute with c11 tag will be rendered as _Noreturn keyword

original issue introduced here: b9a1eb43

Avoid warning about missing proto during typechecking.
Also changes a warning into an error, and improves the error message.

In order to avoid an undefined behavior in the C code generated for this macro
(conversion from an absolute integer to a pointer).