fuzzer: modernize FuzzedDataProvider conversions (#177794)

This change modernizes FuzzedDataProvider.h now that C++17+ is standard
in LLVM.
Replace the runtime if with if constexpr in ConvertUnsignedToSigned
Make the unsigned/signed comparison explicit by casting TS::max() to TU

NOKEYCHECK=True
GitOrigin-RevId: 1ce7a8159352e903f1dab50ff340fcd32a84cf63
1 file changed