Disable another useless warning on MSVC.

Cherry-picked from ff691be03cdde9aa1284096d37f02865dc698eeb in
compatibility branch.
Disable "unreachable code" warnings on MSVC.

Every assertion has `return` after std::abort() to ensure that the code
will still compile when CORRADE_GRACEFUL_ASSERT is set. When it is not
set, the return really *is* unreachable, but we can't do anything about
that.