More careful ClassTag instantiation

We now use a blend of the new scheme and backwards compatible special case if type variables
as ClassTag arguments are constrained by further type variables.

Fixes scala#23611

[Cherry-picked b677f97]

Author: odersky

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

@@ -4985,11 +4985,17 @@ object Types extends TypeUtils {
      */
     private def currentEntry(using Context): Type = ctx.typerState.constraint.entry(origin)
 
+    /** For uninstantiated type variables: the lower bound */
+    def lowerBound(using Context): Type = currentEntry.loBound
+
+    /** For uninstantiated type variables: the upper bound */
+    def upperBound(using Context): Type = currentEntry.hiBound
+
     /** For uninstantiated type variables: Is the lower bound different from Nothing? */
-    def hasLowerBound(using Context): Boolean = !currentEntry.loBound.isExactlyNothing
+    def hasLowerBound(using Context): Boolean = !lowerBound.isExactlyNothing
 
     /** For uninstantiated type variables: Is the upper bound different from Any? */
-    def hasUpperBound(using Context): Boolean = !currentEntry.hiBound.isTopOfSomeKind
+    def hasUpperBound(using Context): Boolean = !upperBound.isTopOfSomeKind
 
     /** Unwrap to instance (if instantiated) or origin (if not), until result
      *  is no longer a TypeVar