Update to bitflags 2 in the compiler

This involves lots of breaking changes. There are two big changes that
force changes. The first is that the bitflag types now don't
automatically implement normal derive traits, so we need to derive them
manually.

Additionally, bitflags now have a hidden inner type by default, which
breaks our custom derives. The bitflags docs recommend using the impl
form in these cases, which I did.

compiler/rustc_ast/src/ast.rs

@@ -2171,9 +2171,10 @@ pub enum InlineAsmRegOrRegClass {
     RegClass(Symbol),
 }
 
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Encodable, Decodable, HashStable_Generic)]
+pub struct InlineAsmOptions(u16);
 bitflags::bitflags! {
-    #[derive(Encodable, Decodable, HashStable_Generic)]
-    pub struct InlineAsmOptions: u16 {
+    impl InlineAsmOptions: u16 {
         const PURE            = 1 << 0;
         const NOMEM           = 1 << 1;
         const READONLY        = 1 << 2;
@@ -2186,6 +2187,12 @@ bitflags::bitflags! {
     }
 }
 
+impl std::fmt::Debug for InlineAsmOptions {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        bitflags::parser::to_writer(self, f)
+    }
+}
+
 #[derive(Clone, PartialEq, Encodable, Decodable, Debug, Hash, HashStable_Generic)]
 pub enum InlineAsmTemplatePiece {
     String(String),