auto merge of #15403 : vhbit/rust/ios-llvm-fixme, r=pcwalton

auto merge of #15403 : vhbit/rust/ios-llvm-fixme, r=pcwalton

It was required to get iOS compilable but since
that time a couple of changes were introduced
which cause the same bug to re-appear and broke
build anyway. Fixing all of them doesn’t look a
viable alternative to me as it will pollute the
code too much.

So it should be fixed from LLVM side and I hope
LLVM will upstream corresponding changes in a
month.

Meanwhile, who wants to play with Rust on iOS is
better to use a fork which uses patched LLVM:
https://github.com/vhbit/rust/tree/ios . It may
lag behind master a bit, but it is Travis-checked
to compile successfully.