Replace the 'final' keyword with 'LLVM_FINAL' as GCC 4.6 doesn't support
the keyword.

If anyone cares deeply about the final optimizations with GCC, we might
want to add a version test to the LLVM definition, but I'm perfectly
happy with this just only firing under Clang and modern MSVC builds.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172898 91177308-0d34-0410-b5e6-96231b3b80d8