Add fixme for removing LlvmArchiveBuilder in the future

Author: bjorn3

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -106,6 +106,10 @@ pub struct LlvmArchiveBuilderBuilder;
 
 impl ArchiveBuilderBuilder for LlvmArchiveBuilderBuilder {
     fn new_archive_builder<'a>(&self, sess: &'a Session) -> Box<dyn ArchiveBuilder + 'a> {
+        // Keeping LlvmArchiveBuilder around in case of a regression caused by using
+        // ArArchiveBuilder.
+        // FIXME remove a couple of months after #128936 gets merged in case no
+        // regression is found.
         if false {
             Box::new(LlvmArchiveBuilder { sess, additions: Vec::new() })
         } else {