[X86] Cleanup/add namespace closure comments. NFCI.

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

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -1238,7 +1238,7 @@ namespace CU {
     UNWIND_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF
   };
 
-} // end CU namespace
+} // namespace CU
 
 class DarwinX86AsmBackend : public X86AsmBackend {
   const MCRegisterInfo &MRI;

llvm/lib/Target/X86/MCTargetDesc/X86MachObjectWriter.cpp

@@ -68,7 +68,7 @@ public:
                           FixedValue);
   }
 };
-}
+} // namespace
 
 static bool isFixupKindRIPRel(unsigned Kind) {
   return Kind == X86::reloc_riprel_4byte ||

llvm/lib/Target/X86/MCTargetDesc/X86ShuffleDecode.cpp

@@ -568,4 +568,4 @@ void DecodeVPERMV3Mask(ArrayRef<uint64_t> RawMask, const APInt &UndefElts,
   }
 }
 
-} // llvm namespace
+} // namespace llvm

llvm/lib/Target/X86/MCTargetDesc/X86WinCOFFStreamer.cpp

@@ -58,7 +58,7 @@ void X86WinCOFFStreamer::finishImpl() {
 
   MCWinCOFFStreamer::finishImpl();
 }
-}
+} // namespace
 
 MCStreamer *llvm::createX86WinCOFFStreamer(MCContext &C,
                                            std::unique_ptr<MCAsmBackend> &&AB,