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_codegen_ssa/src/lib.rs

@@ -110,6 +110,7 @@ pub enum ModuleKind {
 }
 
 bitflags::bitflags! {
+    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
     pub struct MemFlags: u8 {
         const VOLATILE = 1 << 0;
         const NONTEMPORAL = 1 << 1;