[MC] Make *ObjectWriter final

Summary:
to make it clear that target customization should be done by inheriting
from MCObjectTargetWriter subclasses instead of MCObjectWriter
subclasses.

Test Plan: 

Reviewers: 

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Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251094

Author: MaskRay

llvm/include/llvm/MC/MCELFObjectWriter.h

@@ -154,7 +154,7 @@ class MCELFObjectTargetWriter : public MCObjectTargetWriter {
   }
 };
 
-class ELFObjectWriter : public MCObjectWriter {
+class ELFObjectWriter final : public MCObjectWriter {
   unsigned ELFHeaderEFlags = 0;
 
 public:

llvm/include/llvm/MC/MCMachObjectWriter.h

@@ -83,7 +83,7 @@ class MCMachObjectTargetWriter : public MCObjectTargetWriter {
   /// @}
 };
 
-class MachObjectWriter : public MCObjectWriter {
+class MachObjectWriter final : public MCObjectWriter {
 public:
   struct DataRegionData {
     MachO::DataRegionType Kind;

llvm/include/llvm/MC/MCWinCOFFObjectWriter.h

@@ -45,7 +45,7 @@ class MCWinCOFFObjectTargetWriter : public MCObjectTargetWriter {
 
 class WinCOFFWriter;
 
-class WinCOFFObjectWriter : public MCObjectWriter {
+class WinCOFFObjectWriter final : public MCObjectWriter {
   friend class WinCOFFWriter;
 
   std::unique_ptr<MCWinCOFFObjectTargetWriter> TargetObjectWriter;