Fix Conv.cpp unary minus operator applied to unsigned type

Summary: Cast from unsigned to signed *before* negating. The resulting assembly code is the same, but there's no warning.

Reviewed By: yfeldblum, jdonald

Differential Revision: D19588115

fbshipit-source-id: d0d61ab0543211e2399f42a86a7a7a2366679d37

folly/Conv.cpp

@@ -489,7 +489,14 @@ class SignedValueHandler<T, true> {
   Expected<T, ConversionCode> finalize(U value) {
     T rv;
     if (negative_) {
+      FOLLY_PUSH_WARNING
+      FOLLY_MSVC_DISABLE_WARNING(4146)
+
+      // unary minus operator applied to unsigned type, result still unsigned
       rv = T(-value);
+
+      FOLLY_POP_WARNING
+
       if (UNLIKELY(rv > 0)) {
         return makeUnexpected(ConversionCode::NEGATIVE_OVERFLOW);
       }