Enable LLVM_FINAL, LLVM_OVERRIDE and LLVM_HAS_VARIADIC_TEMPLATES with more gcc and MSVC versions

The 'sealed' definition of LLVM_FINAL can be dropped once VS 2010 is
decommissioned.

Some of this is speculative so will keep an eye on the waterfall -- ping me if
you see failures.

Incremental work towards C++11 migration.

llvm-svn: 198399