[clangd] Extract symbol-scope logic out of Quality, add tests. NFC

This prepares for reuse from the semantic highlighting code.

There's a bit of yak-shaving here:
 - when the enum is moved into the clangd namespace, promote it to a
   scoped enum. This means teaching the decision forest infrastructure
   to deal with scoped enums.
 - AccessibleScope isn't quite the right name: e.g. public class members
   are treated as accessible, but still have class scope. So rename to
   SymbolScope.
 - Rename some QualitySignals members to avoid name conflicts.
   (the string) SymbolScope -> Scope
   (the enum) Scope -> ScopeKind

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