Revert "Added Builtin.unreachable() calls (swiftlang#61422)"

This reverts commit 5c3459f.

Author: greenthings

stdlib/public/core/Mirror.swift

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