Support -Wsign-compare compilation

Support -Wsign-compare compilation

Summary:
Due to how Conv.h uses the less_than template inside
not-actually-static static ifs clang still ends up validating generated
code inside a FOLLY_RANGE_CHECK which is never executed due to the
templated types. This code however still generates -Wsign-compare issues
so suppress that in order to allow includers to use this flag.

A simple example will illustrate this:

uint64_t foo = 1;
int bar = folly::to<int>(foo);

Test Plan: fbmake runtests

Reviewed By: meyering@fb.com, njormrod@fb.com

Subscribers: trunkagent, folly-diffs@, bmatheny, ranjeeth, subodh, kmdent, fma, shikong, pgriess, jdelong

FB internal diff: D1731411

Signature: t1:1731411:1418243200:ed1f34a1485669c9cb18f9f6029aca70e498953c