`noreturn` attributes have been added to the mingw-w64 headers, meaning
that from 11.0.0 onwards, you'll no-longer see `-Wreturn-type` warnings
when using assert(false):
1690994f51.
Add -Wno-return-type to the Windows CI, where is should have been all
along, and document why it's required. This can be dropped when we are
using the fixed version of the mingw-w64 headers there.
Drop the -Werror -Wno-return-type special case from our build system.
-Wreturn-type is on by default in Clang and GCC.