Clang 3.9.0 segfaults with variant/using-declaration

[springmeyer]

This ticket is to document that mapbox::variant usage may trigger a segfault in the recent clang 3.9.0 release. There is unlikely anything we need to do in mapbox::variant, but I'd like to keep this open as a notice until the issue is resolved in clang.

• Original mapnik usage of variant that uncovered the problem: mapnik/mapnik#3507
• llvm bug tracker: https://llvm.org/bugs/show_bug.cgi?id=30274

[springmeyer]

Bug against clang is being discussed at https://llvm.org/bugs/show_bug.cgi?id=29087. Since this is a clang regression there appears to be nothing we need to do on our end. Closing.