* debian/control: build-depend on clang-8 and llvm-8, which are now in xenial-updates
* debian/rules: build gn with clang 8
* debian/patches/constexpr-errors-with-old-clang.patch: removed, no longer needed
* debian/patches/fix-extra-arflags.patch: removed, no longer needed
* debian/patches/stl-util-old-clang-compatibility.patch: removed, no longer needed
* debian/patches/suppress-newer-clang-warning-flags.patch: updated
* debian/patches/use-clang-versioned.patch: updated