Remember mutability in DefKind::Static.

This allows to compute the `BodyOwnerKind` from `DefKind` only, and
removes a direct dependency of some MIR queries onto HIR.

As a side effect, it also simplifies metadata, since we don't need 4
flavours of `EntryKind::*Static` any more.

compiler/rustc_middle/src/ty/mod.rs

@@ -2103,7 +2103,7 @@ impl<'tcx> TyCtxt<'tcx> {
         match instance {
             ty::InstanceDef::Item(def) => match self.def_kind(def.did) {
                 DefKind::Const
-                | DefKind::Static
+                | DefKind::Static(..)
                 | DefKind::AssocConst
                 | DefKind::Ctor(..)
                 | DefKind::AnonConst