Fix issue with pc breaking in requiredMethod on newly overloaded valueOf

[jchyb]

We recently started having issues when running completions in metals, with the presentation compiler crashing:

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.

[jchyb]

ValueOf.valueOf does not exist and going by the comment, it seems that the ValueOf class was meant to be filtered here instead, so I changed this.

[tgodzik]

LGTM! Thanks!