chore: import Std.Data.HashMap in backported files

Author: kim-em

src/Lean/Elab/BuiltinCommand.lean

@@ -12,6 +12,7 @@ import Lean.Elab.Eval
 import Lean.Elab.Command
 import Lean.Elab.Open
 import Lean.Elab.SetOption
+import Std.Data.HashMap
 
 namespace Lean.Elab.Command
 

src/Lean/Elab/MutualDef.lean

@@ -16,6 +16,7 @@ import Lean.Elab.Deriving.Basic
 import Lean.Elab.PreDefinition.Main
 import Lean.Elab.PreDefinition.TerminationHint
 import Lean.Elab.DeclarationRange
+import Std.Data.HashMap
 
 namespace Lean.Elab
 open Lean.Parser.Term