Implementation of repr struct alignment RFC 1358.

The main changes around rustc::ty::Layout::struct and rustc_trans:adt:
* Added primitive_align field which stores alignment before repr align
* Always emit field padding when generating the LLVM struct fields
* Added methods for adjusting field indexes from the layout index to the
  LLVM struct field index

The main user of this information is rustc_trans::adt::struct_llfields
which determines the LLVM fields to be used by LLVM, including padding
fields.

src/libsyntax_ext/deriving/generic/mod.rs

@@ -773,7 +773,7 @@ fn find_repr_type_name(diagnostic: &Handler, type_attrs: &[ast::Attribute]) -> &
     for a in type_attrs {
         for r in &attr::find_repr_attrs(diagnostic, a) {
             repr_type_name = match *r {
-                attr::ReprPacked | attr::ReprSimd => continue,
+                attr::ReprPacked | attr::ReprSimd | attr::ReprAlign(_) => continue,
                 attr::ReprExtern => "i32",
 
                 attr::ReprInt(attr::SignedInt(ast::IntTy::Is)) => "isize",