Added Builtin.unreachable() calls (#61422)

Previously unavailable due to #57622 (fixed)

Author: jakedves

stdlib/public/core/Mirror.swift

@@ -482,26 +482,23 @@ public struct Mirror {
     displayStyle: DisplayStyle? = nil,
     ancestorRepresentation: AncestorRepresentation = .generated
   ) where C.Element == Child {
-    // FIXME: Can't use Builtin.unreachable() due to https://github.com/apple/swift/issues/57622.
-    self.init(reflecting: subject)
+    Builtin.unreachable()
   }
   public init<Subject, C: Collection>(
     _ subject: Subject,
     unlabeledChildren: C,
     displayStyle: DisplayStyle? = nil,
     ancestorRepresentation: AncestorRepresentation = .generated
   ) {
-    // FIXME: Can't use Builtin.unreachable() due to https://github.com/apple/swift/issues/57622.
-    self.init(reflecting: subject)
+    Builtin.unreachable()
   }
   public init<Subject>(
     _ subject: Subject,
     children: KeyValuePairs<String, Any>,
     displayStyle: DisplayStyle? = nil,
     ancestorRepresentation: AncestorRepresentation = .generated
   ) {
-    // FIXME: Can't use Builtin.unreachable() due to https://github.com/apple/swift/issues/57622.
-    self.init(reflecting: subject)
+    Builtin.unreachable()
   }
   public let subjectType: Any.Type
   public let children: Children