add #[align] attribute

Right now it's used for functions with `fn_align`, in the future it will
get more uses (statics, struct fields, etc.)

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -182,6 +182,9 @@ impl Deprecation {
 #[derive(Clone, Debug, HashStable_Generic, Encodable, Decodable, PrintAttribute)]
 pub enum AttributeKind {
     // tidy-alphabetical-start
+    /// Represents `#[align(N)]`.
+    Align { align: Align, span: Span },
+
     /// Represents `#[rustc_allow_const_fn_unstable]`.
     AllowConstFnUnstable(ThinVec<Symbol>),