Removed LLVM bug workaround

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.