Fix issue with pc breaking in requiredMethod on newly overloaded valueOf (scala#23708)

We recently started having issues when running completions in metals,
with the presentation compiler crashing:
```scala
dotty.tools.dotc.core.TypeError$.apply(TypeErrors.scala:54)
	dotty.tools.dotc.core.Denotations$MultiDenotation.suchThat(Denotations.scala:1280)
	dotty.tools.dotc.core.Denotations$Denotation.requiredSymbol(Denotations.scala:305)
	dotty.tools.dotc.core.Denotations$Denotation.requiredMethod(Denotations.scala:321)
	dotty.tools.pc.completions.Completions.isUninterestingSymbol$lzyINIT1(Completions.scala:744)
	dotty.tools.pc.completions.Completions.isUninterestingSymbol(Completions.scala:725)
	dotty.tools.pc.completions.Completions.includeSymbol(Completions.scala:90)
	dotty.tools.pc.completions.Completions.visit$3(Completions.scala:697)
	dotty.tools.pc.completions.Completions.filterInteresting$$anonfun$1(Completions.scala:715)
	scala.collection.immutable.List.foreach(List.scala:334)
	dotty.tools.pc.completions.Completions.filterInteresting(Completions.scala:715)
	dotty.tools.pc.completions.Completions.enrichedCompilerCompletions(Completions.scala:118)
	dotty.tools.pc.completions.Completions.completions(Completions.scala:136)
	dotty.tools.pc.completions.CompletionProvider.completions(CompletionProvider.scala:139)
	dotty.tools.pc.ScalaPresentationCompiler.complete$$anonfun$1(ScalaPresentationCompiler.scala:194)


dotty.tools.dotc.core.TypeError$$anon$1: Failure to disambiguate overloaded reference with
  method valueOf in object Predef: [T]: T  and
  method valueOf in object Predef: [T](implicit vt: ValueOf[T]): T
```
This happened after the recent changes to the stdlib, with the, I
believe, newly added valueOf overload, and the previously used
`requiredMethod` by design not handling these cases.

I also noticed that in the same piece of code we had a bit of an empty
Symbol factory situation going on, with MultiDenotation being changed
into a Symbol (always resulting in an empty symbol), and only later
flattened with the `alternatives`, so I changed that too.

I can't really test this properly, as the pc tests seem to use an older
stdlib, but at least the `wait` methods do resolve properly after these
changes, so I have no reason to think the valueOf methods would be any
different.
[Cherry-picked 1b5e93b]

Author: jchyb

presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -736,15 +736,18 @@ class Completions(
     defn.Object_notifyAll,
     defn.Object_notify,
     defn.Predef_undefined,
-    defn.ObjectClass.info.member(nme.wait_).symbol,
     // NOTE(olafur) IntelliJ does not complete the root package and without this filter
     // then `_root_` would appear as a completion result in the code `foobar(_<COMPLETE>)`
     defn.RootPackage,
     // NOTE(gabro) valueOf was added as a Predef member in 2.13. We filter it out since is a niche
     // use case and it would appear upon typing 'val'
-    defn.ValueOfClass.info.member(nme.valueOf).symbol,
-    defn.ScalaPredefModule.requiredMethod(nme.valueOf)
-  ).flatMap(_.alternatives.map(_.symbol)).toSet
+    defn.ValueOfClass
+  ) ++ (
+    Set(
+      defn.ObjectClass.info.member(nme.wait_),
+      defn.ScalaPredefModule.info.member(nme.valueOf)
+    ).flatMap(_.alternatives.map(_.symbol)).toSet
+  )
 
   private def isNotLocalForwardReference(sym: Symbol)(using Context): Boolean =
     !sym.isLocalToBlock ||

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -128,9 +128,6 @@ class CompletionSuite extends BaseCompletionSuite:
          |isInstanceOf[X0]: Boolean
          |synchronized[X0](x$0: X0): X0
          |toString(): String
-         |wait(): Unit
-         |wait(x$0: Long): Unit
-         |wait(x$0: Long, x$1: Int): Unit
          |""".stripMargin
     )