Support symbol literals under -Xsource:3

Scala 3 still supports symbol literals even if they require a language
import now (cf scala/scala3#11588), so don't
emit an error if we find one under -Xsource:3 as that could
unnecessarily impede cross-compilation as discovered in
scala/scala-dev#769.

Author: smarter

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -1421,8 +1421,7 @@ self =>
       else if (in.token == SYMBOLLIT) {
         def msg(what: String) =
           s"""symbol literal is $what; use Symbol("${in.strVal}") instead"""
-        if (settings.isScala3) syntaxError(in.offset, msg("unsupported"))
-        else deprecationWarning(in.offset, msg("deprecated"), "2.13.0")
+        deprecationWarning(in.offset, msg("deprecated"), "2.13.0")
         Apply(scalaDot(nme.Symbol), List(finish(in.strVal)))
       }
       else finish(in.token match {