[SwiftLexicalLookup] Add dollar identifier, better extension handling and more `ASTScope` related fixes

[MAJKFL]

This PR brings more ASTScope related fixes to SwiftLexicalLookup. Those include:

• Prompting clients to lookup generic parameters of extended types inside extensions with LookupResult.lookInGenericParametersOfExtendedType.
• Dollar identifier handling through LookupResult.mightIntroduceDollarIdentifiers result kind when looking up for identifier = nil and more explicit LookupName.dollarIdentifier when looking up for a particular dollar identifier.
• Remove member and file scope handling to avoid confusion and incomplete member lookup.
• Fix behavior of lookup from where clauses.
• And more fine grained fixes.

[MAJKFL]

@swift-ci Please test

[DougGregor]

@swift-ci please test Windows

[MAJKFL]

@swift-ci Please test

[DougGregor]

@swift-ci please test Windows

[DougGregor]

@swift-ci please test Windows