Upgrading to clang++ 7.x

[springmeyer]

It would be nice to be able to use the latest version of clang++. However we'll hit:

In file included from ../src/vtshaver.cpp:1:
In file included from ../src/filters.hpp:4:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/style/filter.hpp:4:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/util/feature.hpp:3:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/util/optional.hpp:3:
/Users/danespringmeyer/projects/vtshaver/mason_packages/osx-x86_64/clang++/7.0.0/include/c++/v1/experimental/optional:11:2: error: "<experimental/optional> has been removed. Use
      <optional> instead."
#error "<experimental/optional> has been removed. Use <optional> instead."

So, we'll need to upgrade mbgl to avoid this (#9) to a version that includes mapbox/mapbox-gl-native#13049