Fix memory leak through NoDenotation.cachePrefix

We may set `NoDenotation.cachePrefix` in `Denotation.asSeenFrom`, thus
leak memory.

This memory leak is discovered by @mlachkar.

Co-authored-by: Meriam Lachkar <[email protected]>

Author: liufengyun

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

@@ -130,7 +130,7 @@ object Denotations {
 
     /** The denotation with info(s) as seen from prefix type */
     final def asSeenFrom(pre: Type)(using Context): AsSeenFromResult =
-      if (Config.cacheAsSeenFrom) {
+      if (Config.cacheAsSeenFrom && exists) {
         if ((cachedPrefix ne pre) || ctx.period != validAsSeenFrom) {
           cachedAsSeenFrom = computeAsSeenFrom(pre)
           cachedPrefix = pre