[MC] Constify MCELF::GetVisibility and MCELF::getOther

These two routines didn't take a "const MCSymbolData &SD"
like the other MCELF::Get routines for some reason ...
Add nonnull in CodeGen for __attribute__((nonnull))

We now have an LLVM-level nonnull attribute that can be applied to function
parameters, and we emit it for reference types (as of r209723), but did not
emit it when an __attribute__((nonnull)) was provided. Now we will.