Reapply r349876:

Apply a workaround to be able to build clang 8.0.0 headers with clang
3.4.1, which is still in the stable/10 branch.

It looks like clang 3.4.1 implements static_asserts by instantiating a
temporary static object, and if those are in an anonymous union, it
results in "error: anonymous union can only contain non-static data
members".

To work around this implementation limitation, move the static_asserts
in question out of the anonymous unions.

This should make building the latest stable/11 from stable/10 possible
again.

Reported by: Mike Tancsa <mike@sentex.net>