Update the TastyFormat for matches with sub-cases

There was a discussion between the alternatives of:
- modifying the tasty format, under the assumption that the tasty in question
  _should_ only be emitted in an experimental tasty version;
- using an annotation to avoid changes to the tasty format.

This PR implements the 1st option, see scala#23810 for the second option.

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -590,7 +590,7 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
             if (tree.isInline)
               if (selector.isEmpty) writeByte(IMPLICIT)
               else { writeByte(INLINE); pickleTree(selector) }
-            else if tree.isSubMatch then { writeByte(LAZY); pickleTree(selector) }
+            else if tree.isSubMatch then { writeByte(SUBMATCH); pickleTree(selector) }
             else pickleTree(selector)
             tree.cases.foreach(pickleTree)
           }

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1551,7 +1551,7 @@ class TreeUnpickler(reader: TastyReader,
                   readByte()
                   InlineMatch(readTree(), readCases(end))
                 }
-                else if nextByte == LAZY then // similarly to InlineMatch we use an arbitrary Cat.1 tag
+                else if nextByte == SUBMATCH then
                   readByte()
                   SubMatch(readTree(), readCases(end))
                 else Match(readTree(), readCases(end)))

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -104,7 +104,7 @@ Standard-Section: "ASTs" TopLevelStat*
                   INLINED        Length expr_Term call_Term? ValOrDefDef*          -- Inlined code from call, with given body `expr` and given bindings
                   LAMBDA         Length meth_Term target_Type?                     -- Closure over method `f` of type `target` (omitted id `target` is a function type)
                   IF             Length [INLINE] cond_Term then_Term else_Term     -- inline? if cond then thenPart else elsePart
-                  MATCH          Length (IMPLICIT | [INLINE] sel_Term) CaseDef*    -- (inline? sel | implicit) match caseDefs
+                  MATCH          Length (IMPLICIT | [INLINE | SUBMATCH] sel_Term) CaseDef* -- ((inline | if)? sel | implicit) match caseDefs
                   TRY            Length expr_Term CaseDef* finalizer_Term?         -- try expr catch {casdeDef} (finally finalizer)?
                   RETURN         Length meth_ASTRef expr_Term?                     -- return expr?,  `methASTRef` is method from which is returned
                   WHILE          Length cond_Term body_Term                        -- while cond do body
@@ -511,6 +511,7 @@ object TastyFormat {
   final val EMPTYCLAUSE = 45
   final val SPLITCLAUSE = 46
   final val TRACKED = 47
+  final val SUBMATCH = 48 // experimental.subCases
 
   // Tree Cat. 2:    tag Nat
   final val firstNatTreeTag = SHAREDterm