[X86] Add missing namespace closure comments. NFCI.

Fixes some clang-tidy llvm-namespace-comment warnings.

llvm/lib/Target/X86/Disassembler/X86Disassembler.cpp

@@ -1662,9 +1662,9 @@ namespace X86 {
     sib   = 504,
     sib64 = 505
   };
-}
+} // namespace X86
 
-}
+} // namespace llvm
 
 static bool translateInstruction(MCInst &target,
                                 InternalInstruction &source,
@@ -1689,7 +1689,7 @@ private:
   DisassemblerMode              fMode;
 };
 
-}
+} // namespace
 
 X86GenericDisassembler::X86GenericDisassembler(
                                          const MCSubtargetInfo &STI,

llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp

@@ -344,4 +344,4 @@ void DecodeVPERMV3Mask(const Constant *C, unsigned ElSize, unsigned Width,
     ShuffleMask.push_back(Index);
   }
 }
-} // llvm namespace
+} // namespace llvm

llvm/lib/Target/X86/X86WinEHState.cpp

@@ -109,7 +109,7 @@ private:
   /// The linked list node subobject inside of RegNode.
   Value *Link = nullptr;
 };
-}
+} // namespace
 
 FunctionPass *llvm::createX86WinEHStatePass() { return new WinEHStatePass(); }