Make room for more than just `Function::isMaterializable()` in the
`GlobalObject` subclass data bitfield. Since we're treating it like a
bitfield, change `Function::Function()` to zero-out the whole thing.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235770 91177308-0d34-0410-b5e6-96231b3b80d8