Only pickle StableRealizable flag for methods

Fixes scala#9276

That aligns behavior with the TastyFormat doc, where it says:

    STABLE  -- Method that is assumed to be stable, i.e. its applications are legal paths

For other occurrences of StableRealizable, we either reconstitute them if they are implied
(i.e. for module vals and enum values), or we recompute them in realizability checking as needed.

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -300,6 +300,11 @@ object Flags {
    *
    *  - the purity analysis used by the inliner to decide whether it is safe to elide, and
    *  - the TASTy reader of Scala 2.13, to determine whether there is a $init$ method.
+   *
+   *  StableRealizable is
+   *  - asserted for methods
+   *  - automatic in conjunction with Module or Enum vals
+   *  - cached for other vals
    */
   val (_, StableRealizable @ _, _) = newFlags(24, "<stable>")
 
@@ -584,6 +589,7 @@ object Flags {
   val MethodOrModule: FlagSet                = Method | Module
   val ParamForwarder: FlagSet                = Method | ParamAccessor | StableRealizable      // A parameter forwarder
   val PrivateMethod: FlagSet                 = Method | Private
+  val StableMethod: FlagSet                  = Method | StableRealizable
   val NoInitsInterface: FlagSet              = NoInits | PureInterface
   val NoInitsTrait: FlagSet                  = NoInits | Trait                                // A trait that does not need to be initialized
   val ValidForeverFlags: FlagSet             = Package | Permanent | Scala2SpecialFlags

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

@@ -739,7 +739,7 @@ class TreePickler(pickler: TastyPickler) {
       if (flags.is(Accessor)) writeModTag(FIELDaccessor)
       if (flags.is(CaseAccessor)) writeModTag(CASEaccessor)
       if (flags.is(HasDefault)) writeModTag(HASDEFAULT)
-      if (flags.is(StableRealizable)) writeModTag(STABLE)
+      if flags.isAllOf(StableMethod) then writeModTag(STABLE) // other StableRealizable flag occurrences are either implied or can be recomputed
       if (flags.is(Extension)) writeModTag(EXTENSION)
       if (flags.is(ParamAccessor)) writeModTag(PARAMsetter)
       if (flags.is(SuperParamAlias)) writeModTag(PARAMalias)

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

@@ -487,7 +487,9 @@ class TreeUnpickler(reader: TastyReader,
       if (lacksDefinition && tag != PARAM) flags |= Deferred
       if (tag == DEFDEF) flags |= Method
       if (givenFlags.is(Module))
-        flags = flags | (if (tag == VALDEF) ModuleValCreationFlags else ModuleClassCreationFlags)
+        flags |= (if (tag == VALDEF) ModuleValCreationFlags else ModuleClassCreationFlags)
+      if flags.is(Enum, butNot = Method) && name.isTermName then
+        flags |= StableRealizable
       if (ctx.owner.isClass) {
         if (tag == TYPEPARAM) flags |= Param
         else if (tag == PARAM) {