scripted-diff: Use [[nodiscard]] (C++17) instead of NODISCARD

-BEGIN VERIFY SCRIPT-
sed -i "s/NODISCARD/[[nodiscard]]/g" $(git grep -l "NODISCARD" ":(exclude)src/bench/nanobench.h" ":(exclude)src/attributes.h")
-END VERIFY SCRIPT-

src/util/strencodings.cpp

@@ -280,7 +280,7 @@ std::string DecodeBase32(const std::string& str, bool* pf_invalid)
     return std::string((const char*)vchRet.data(), vchRet.size());
 }
 
-NODISCARD static bool ParsePrechecks(const std::string& str)
+[[nodiscard]] static bool ParsePrechecks(const std::string& str)
 {
     if (str.empty()) // No empty string allowed
         return false;