[Analysis] Fix a build error regarding std::variant

This patch fixes:

  llvm/include/llvm/Analysis/LoopAccessAnalysis.h:381:8: error: no
  template named 'variant' in namespace 'std'

Author: kazutakahirata

llvm/include/llvm/Analysis/LoopAccessAnalysis.h

@@ -19,6 +19,7 @@
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/IR/DiagnosticInfo.h"
 #include <optional>
+#include <variant>
 
 namespace llvm {