Ignore `-Wsign-conversion` issues

clang++ compilation when `-Wsign-conversion` is currently broken and the
issues within the code are varied and widespread. For the time being
ignore `-Wsign-conversion` issues, even though some of them are valid
and bleed over into issues that would be found with
`-Wtautological-compare`, et al.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>