[ObjC] Parse sections containing Objective-C constants

[bdash]

This adds support for the __objc_arrayobj, _objc_dictobj, __objc_intobj, __objc_floatobj, __objc_doubleobj and __objc_dateobj sections that contain Objective-C constants. These are emitted by Apple's versions of Clang for const literals, amongst other things.

There are a few points here where I'm not sure what the best approach is:

1. Should there be a new setting or settings to control parsing of these sections? For now I've keyed it off the setting for CFString, but if it stays like that the setting description will need to be updated.
2. How should the symbols for the variables in these sections be named?
3. Does it make sense to create a single SymbolQueue and share it between all of these functions rather than having each create its own?
4. Should this code really be doing the BeginUndoActions / ForgetUndoActions dance? I copied it from ProcessCFStrings. IIRC that's problematic for performance due to needing to synchronize with the main thread, and doesn't make a lot of sense for background work like this.
5. Any suggestions on how to avoid some of the duplication in this code would be appreciated. Sharing the SymbolQueue would simplify things slightly.
6. This code can throw exceptions, but doesn't seem to be particularly exception safe. The SymbolQueue is a member variable that's a raw pointer and will leak if an exception is thrown. An exception after BeginBulkModifySymbols is called will leave that in an unbalanced state. These are issues are all carried over from ProcessCFString.

A couple of libraries that are good for testing with in the macOS shared cache:

1. ImageIO has arrays, dictionaries, floats, and doubles.
2. libXTFontStaticRegistryData.dylib has integers, dates, and data constants.