[AArch64, Lanai] Fix some Clang-tidy modernize and Include What You Use warnings; other minor fixes (NFC).

llvm-svn: 291197
diff --git a/llvm/lib/Target/Lanai/Disassembler/LanaiDisassembler.h b/llvm/lib/Target/Lanai/Disassembler/LanaiDisassembler.h
index a317cd8..e0c19e8 100644
--- a/llvm/lib/Target/Lanai/Disassembler/LanaiDisassembler.h
+++ b/llvm/lib/Target/Lanai/Disassembler/LanaiDisassembler.h
@@ -20,14 +20,11 @@
 
 namespace llvm {
 
-class MCInst;
-class raw_ostream;
-
 class LanaiDisassembler : public MCDisassembler {
 public:
   LanaiDisassembler(const MCSubtargetInfo &STI, MCContext &Ctx);
 
-  ~LanaiDisassembler() override {}
+  ~LanaiDisassembler() override = default;
 
   // getInstruction - See MCDisassembler.
   MCDisassembler::DecodeStatus
@@ -36,6 +33,6 @@
                  raw_ostream &CStream) const override;
 };
 
-} // namespace llvm
+} // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_LANAI_DISASSEMBLER_LANAIDISASSEMBLER_H