[ObjC] Avoid leaking SymbolQueue

`SymbolQueue`'s lifetime was being managed via `new` / `delete`. This
was error-prone in the face of code that can a) throw exceptions, or b)
return early. Typically the solution would be to move to
`std::unique_ptr` and call it a day, but nothing is ever easy.

The `SymbolQueue`s created by `ObjCProcessor` are intended to live for
the duration of `ProcessObjCData` and `ProcessCFStrings` methods. Since
they are used multiple levels down the call tree, the active
`SymbolQueue` is stored as a member variable on `ObjCProcessor`.
Switching to `std::unique_ptr` would ensure that the `SymbolQueue` is
destroyed, but would leave a dangling pointer in the member variable.

To address this I'm introducing a `ScopedSingleton` class that provides
a thread-local singleton whose lifetime is controlled by a guard object.
`ProcessObjCData` / `ProcessCFStrings` call `ScopedSymbolQueue::Make`
and store the returned guard object in a local. Code that accesses the
symbol queue uses `ScopedSymbolQueue::Get` to retrieve the current
instance. The guard object ensures the `SymbolQueue` is deleted and the
`current` pointer is cleared no matter how the scope is exited.

A better longer-term design is to introduce a class for processing the
Objective-C runtime metadata and a class for processing constants such
as `CFString`s. These could directly own the symbol queue so it would be
accessible to any member functions that define symbols. This is a more
involved refactoring than I have time for right now.

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