[libFuzzer] replace std::random_shuffle with std::shuffle as std::random_shuffle is being deprecated in C++17. Also simplify fuzzer::Random. NFC

llvm-svn: 294366
diff --git a/llvm/lib/Fuzzer/FuzzerCorpus.h b/llvm/lib/Fuzzer/FuzzerCorpus.h
index 468d5e5..8c2f703 100644
--- a/llvm/lib/Fuzzer/FuzzerCorpus.h
+++ b/llvm/lib/Fuzzer/FuzzerCorpus.h
@@ -97,7 +97,7 @@
   // Hypothesis: units added to the corpus last are more likely to be
   // interesting. This function gives more weight to the more recent units.
   size_t ChooseUnitIdxToMutate(Random &Rand) {
-    size_t Idx = static_cast<size_t>(CorpusDistribution(Rand.Get_mt19937()));
+    size_t Idx = static_cast<size_t>(CorpusDistribution(Rand));
     assert(Idx < Inputs.size());
     return Idx;
   }