Will -fsyntax-only hide issues with -pedantic? I think not...

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

The answer to $subject is not at all urgent, but I noticed we can get
some (very modest) speed increases in the "pedantic" CI job when adding
-fsyntax-only to CFLAGS. This currently requires monkeypatching out the
"-o <target> -c" part hardcoded in the Makefile. See cebead1ebfb (ci:
run a pedantic build as part of the GitHub workflow, 2021-08-08) for the
pedantic job.

I.e. I'm aware of CFLAGS's -O<n> changing which warings we emit, but
does -fsyntax-only?

The gcc manpage suggests that it would, saying:

    Check the code for syntax errors, but don't do anything beyond that

Whereas clang's says:

    Run the preprocessor, parser and type checking stages. 

I think gcc's is a case of its docs drifting out of sync with the
implementation. Both will warn on e.g. this program under -pedantic,
which gcc wouldn't be doing if it only did syntax parsing (and didn't
run the warning machinery):
    
    int main(void)
    {
            int v[0];
            return 0;
    }

I don't have any practical use for this now. We could squeeze some
slight performance out of one CI jobs, but perhaps it'll be more
interesting in the future.