fix: don't strip macro scopes in names of specialized LCNF decls (#6930)

This PR changes the name generation of specialized LCNF decls so they
don't strip macro scopes. This avoids name collisions for
specializations created in distinct macro scopes. Since the normal
Name.append function checks for the presence of macro scopes, we need to
use appendCore.

src/Lean/Compiler/LCNF/Specialize.lean

@@ -208,7 +208,7 @@ Specialize `decl` using
 - `levelParamsNew`: the universe level parameters for the new declaration.
 -/
 def mkSpecDecl (decl : Decl) (us : List Level) (argMask : Array (Option Arg)) (params : Array Param) (decls : Array CodeDecl) (levelParamsNew : List Name) : SpecializeM Decl := do
-  let nameNew := decl.name ++ `_at_ ++ (← read).declName.eraseMacroScopes ++ (`spec).appendIndexAfter (← get).decls.size
+  let nameNew := decl.name.appendCore `_at_ |>.appendCore (← read).declName |>.appendCore `spec |>.appendIndexAfter (← get).decls.size
   /-
   Recall that we have just retrieved `decl` using `getDecl?`, and it may have free variable identifiers that overlap with the free-variables
   in `params` and `decls` (i.e., the "closure").