Address clang sign warning in Format-inl.h
Summary:
This has been bothering me for a while when I've been trying to
verify builds with clang. This generates a ton of spew and the current
code can't actually handle non-default negatives so let's just throw if
we see one.
Test Plan: fbmake runtests
Reviewed By: meyering@fb.com
Subscribers: mathieubaudet, njormrod, folly-diffs@, bmatheny, benyluo, shikong, kmdent, fma, ranjeeth, subodh
FB internal diff:
D1643881
Signature: t1:
1643881:
1414523777:
085035ce8c280fbd6481b2ce0354b53b96fdc50e