Fix @experimental @main methods

Fixes #13848

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/MainProxies.scala

@@ -94,8 +94,12 @@ object MainProxies {
       val body = Try(call, handler :: Nil, EmptyTree)
       val mainArg = ValDef(nme.args, TypeTree(defn.ArrayType.appliedTo(defn.StringType)), EmptyTree)
         .withFlags(Param)
+      val experimentalAnnotOpt = mainFun.getAnnotation(defn.ExperimentalAnnot).map { annot =>
+        makeNew(ref(defn.ExperimentalAnnot)).withSpan(annot.tree.span)
+      }
       val mainMeth = DefDef(nme.main, (mainArg :: Nil) :: Nil, TypeTree(defn.UnitType), body)
         .withFlags(JavaStatic)
+        .withAnnotations(experimentalAnnotOpt.toList)
       val mainTempl = Template(emptyConstructor, Nil, Nil, EmptyValDef, mainMeth :: Nil)
       val mainCls = TypeDef(mainFun.name.toTypeName, mainTempl)
         .withFlags(Final | Invisible)

tests/neg-custom-args/no-experimental/i13848.scala

@@ -0,0 +1,7 @@
+import annotation.experimental
+
+@main
+def run(): Unit = f // error
+
+@experimental
+def f = 2

tests/pos-custom-args/no-experimental/i13848.scala

@@ -0,0 +1,8 @@
+import annotation.experimental
+
+@main
+@experimental
+def run(): Unit = f
+
+@experimental
+def f = 2