[randstruct] Use llvm::shuffle to avoid STL impl difference after D121556

This reverts commit 2a2149c.
This reverts commit 8d7595b.
This reverts commit e2e6899.

If this doesn't work, I'll revert the whole thing.

Author: MaskRay

clang/lib/AST/Randstruct.cpp

@@ -150,14 +150,14 @@ void randomizeStructureLayoutImpl(const ASTContext &Context,
   if (CurrentBitfieldRun)
     Buckets.push_back(std::move(CurrentBitfieldRun));
 
-  std::shuffle(std::begin(Buckets), std::end(Buckets), RNG);
+  llvm::shuffle(std::begin(Buckets), std::end(Buckets), RNG);
 
   // Produce the new ordering of the elements from the Buckets.
   SmallVector<FieldDecl *, 16> FinalOrder;
   for (const std::unique_ptr<Bucket> &B : Buckets) {
     llvm::SmallVectorImpl<FieldDecl *> &RandFields = B->fields();
     if (!B->isBitfieldRun())
-      std::shuffle(std::begin(RandFields), std::end(RandFields), RNG);
+      llvm::shuffle(std::begin(RandFields), std::end(RandFields), RNG);
 
     FinalOrder.insert(FinalOrder.end(), RandFields.begin(), RandFields.end());
   }

clang/unittests/AST/CMakeLists.txt

@@ -25,6 +25,7 @@ add_clang_unittest(ASTTests
   EvaluateAsRValueTest.cpp
   ExternalASTSourceTest.cpp
   NamedDeclPrinterTest.cpp
+  RandstructTest.cpp
   RecursiveASTVisitorTest.cpp
   SizelessTypesTest.cpp
   SourceLocationTest.cpp