fix(#16458): regression in xml syntax parsing (#19522)

close #16458
xLiteral mistakenly assumed that the element just after `<` is always
non-special element, but it is not true. It could be xCharData, comment,
xProcInstr.

compiler/src/dotty/tools/dotc/parsing/xml/MarkupParsers.scala

@@ -373,7 +373,7 @@ object MarkupParsers {
           while {
             xSpaceOpt()
             nextch()
-            ts.append(element)
+            content_LT(ts)
             charComingAfter(xSpaceOpt()) == '<'
           } do ()
           handle.makeXMLseq(Span(start, curOffset, start), ts)

tests/run/i16458.scala

@@ -0,0 +1,54 @@
+
+object Test {
+  import scala.xml.*
+  def main(args: Array[String]): Unit = {
+    val xml =  <div>FooBar</div><!-- /.modal-content -->
+    assert(
+      xml match
+        case Seq(elm: Elem, comment: Comment) if
+            elm.label == "div" &&
+            elm.child(0) == Atom(Text("FooBar")) &&
+            comment.label == " /.modal-content "
+              => true
+        case _ => false
+      ,
+      xml
+    )
+  }
+}
+
+package scala.xml {
+  type MetaData = AnyRef
+
+  trait NamespaceBinding
+  object TopScope extends NamespaceBinding
+  object Null
+  abstract class Node {
+    def label: String
+    def child: Seq[Node]
+    override def toString = label + child.mkString
+  }
+  class Comment(commentText: String) extends Node{
+    def label = commentText
+    def child = Nil
+  }
+  class Elem(prefix: String, val label: String, attributes1: MetaData, scope: NamespaceBinding, minimizeEmpty: Boolean, val child: Node*) extends Node
+  class NodeBuffer extends Seq[Node] { 
+    val nodes = scala.collection.mutable.ArrayBuffer.empty[Node]
+    def &+(o: Any): NodeBuffer =
+      o match {
+        case n: Node => nodes.addOne(n) ; this
+        case t: Text => nodes.addOne(Atom(t)) ; this
+      }
+    // Members declared in scala.collection.IterableOnce
+    def iterator: Iterator[scala.xml.Node] = nodes.iterator
+    // Members declared in scala.collection.SeqOps
+    def apply(i: Int): scala.xml.Node = nodes(i)
+    def length: Int = nodes.length
+  }
+  case class Text(text: String)
+  case class Atom(t: Text) extends Node {
+    def label = t.text
+    def child = Nil
+  }
+}