[NFC] Add missing Clang Cache headers

Clang cache uses std::variant, std::unique_ptr, and std::optional. The
missing headers are resulting in build failures.

Author: etcwilde

clang/lib/Frontend/CachedDiagnostics.cpp

@@ -25,6 +25,7 @@
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/YAMLTraits.h"
+#include <variant>
 
 using namespace clang;
 using namespace clang::cas;

clang/lib/Frontend/CachedDiagnostics.h

@@ -11,6 +11,8 @@
 
 #include "clang/Basic/LLVM.h"
 #include "llvm/Support/PrefixMapper.h"
+#include <memory>
+#include <optional>
 
 namespace clang {
 class DiagnosticConsumer;