Regressed in scala#6846, which added support for encoding repeated
annotations.

Test failure before replacing `groupBy` with `LinkedHashMap`:

```
$ sbt junit/testOnly scala.tools.nsc.DeterminismTest
...
java.lang.AssertionError: assertion failed: Difference detected between recompiling List(b.scala, Annot1.java) Run:
jardiff -r /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/reference814657788418452571 /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/recompileOutput4882243280168823330

$ jardiff -r /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/reference814657788418452571 /var/folders/tz/p8vd07wn7wxck3b9v54grlzw0000gp/T/recompileOutput4882243280168823330
diff --git a/Test.class.asm b/Test.class.asm
index 98bfd80..a056f9a 100644
--- a/Test.class.asm
+++ b/Test.class.asm
@@ -4,10 +4,10 @@

   // compiled from: b.scala

-  @LAnnot2;(value=java.lang.Object.class)
-
   @LAnnot1;(value="foo")

+  @LAnnot2;(value=java.lang.Object.class)
+
   @Lscala/reflect/ScalaSignature;(bytes="\u0006\u0001u1AAA\u0002\u0001\r!)Q\u0002\u0001C\u0001\u001d\u0009!A+Z:u\u0015\u0005!\u0011a\u0002\u001ff[B$\u0018PP\u0002\u0001'\u0009\u0001q\u0001\u0005\u0002\u0009\u00175\u0009\u0011BC\u0001\u000b\u0003\u0015\u00198-\u00197b\u0013\u0009a\u0011B\u0001\u0004B]f\u0014VMZ\u0001\u0007y%t\u0017\u000e\u001e \u0015\u0003=\u0001\"\u0001\u0005\u0001\u000e\u0003\rAC\u0001\u0001\n\u0016-A\u0011\u0001cE\u0005\u0003)\r\u0011a!\u00118o_R\u0014\u0014!\u0002<bYV,7%A\u0004)\u0009\u0001ARc\u0007\u0009\u0003!eI!AG\u0002\u0003\r\u0005sgn\u001c;2C\u0005a\u0012a\u00014p_\u0002")
```

WIP stabilize annotation ordering