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>