On x86_64, a proper solution (see GitHub #120) is still needed,
but this will have to wait until the special case for extern(D)
is gone from the implementation.
We need this right now as std.digest.md in 2.061 triggers a
miscompilation issue in the LLVM x86 backend (not the optimizer!)
when returning them directly as LLVM arrays.
The solution is to replace Attribute with AttrBuilder in IrFuncTyArg.
Then the argument attributes can be easily manipulated and transformed
into the final AttributeSet.
