MSVC 2010 build fix

Back out the part of r198399 that enabled LLVM_FINAL/LLVM_OVERRIDE on VS 2010.

DwarfUnit.h legitimately uses them on destructors which unfortunately triggers
Compiler Error C3665 (override specifier not allowed on a destructor/finalizer)
prior to MSVC 2012:

  virtual ~DwarfCompileUnit() LLVM_OVERRIDE;

llvm-svn: 198401