[ffigen] Remove Constant from public AST

[liamappelbe]

Constant is used as the base class of UnnamedEnumConstant and MacroConstant. But it's also used in parseVarDeclaration, when we want to codegen the parsed variable as a Dart constant instead of a Global. This pollutes the public AST with internal details. Constant should not be part of the public AST at all.a

Beyond that API issue, it's also a problem for the visitor config migration because part of that constness decision is checking config.globals.includeSymbolAddress at parse time (if a user wants the symbol address, we need to treat it as a Global instead of a Constant). We want to move that config field to a visitor, so this refactor changes parseVarDeclaration to always return a Global. If the variable has a constant value, we store it on the Global. That lets us defer the decision about how to codegen it.

Generally this simplifies things, but it slightly complicates the Library constructor's decision about whether a binding belongs in lookupBindings or noLookupBindings, since a Global is a LookupBinding in the class hierarchy, but doesn't actually look anything up if isConst.

ConstantValue.type mirrors Constant.rawType. It would be nice if we could use the type of the enclosing Global. But it's not that simple because, for example, const char* Globals are codegenned as Pointer<Char>, but we want to codegen the ConstantValue as String.

[github-actions]

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[goderbauer]

LGTM