Integrate numerical label IDs to Yul

[clonker]

With this change, nodes in the Yul AST are no longer referred to by string name (or more precisely: statically managed repository of strings and referenced by hashes thereof) but by numerical ID. The numerical ID belongs to an immutable ASTLabelRegistry, in which it is associated to a string label. In turn, the registry is owned by the yul::AST.

During, e.g., optimization it is possible to generate new LabelIDs, which not yet have an associated label in a registry but encode a parent label (which may be empty) using the yul::LabelIDDispenser. Newly generated IDs are only valid with respect to their generating instance of LabelIDDispenser.
At the end of optimization, when f.ex. serialization back into a stringified form is required, the LabelIDs currently contained in the AST are assigned new labels by the yul::LabelIDDispenser, generating a new instance of ASTLabelRegistry.

This PR removes YulString, yul::NameSimplifier, yul::NameDispenser, yul::VarNameCleaner and replaces them by the LabelID and related utilities.

There is a special case of ghost nodes. These are introduced in CFGs to map switch statements to sequences of if-else blocks but do not live in the actual AST, just the CFG. To this end, a CFG also holds an instance of ASTLabelRegistry and manages the LabelID -> string_view transformation itself. An ASTLabelRegistry itself does not know how to stringify the special ghost nodes.

Depends on #15838.

[aarlt]

Just read through this. Two things so far:

1. not strictly related to this PR, but I think its not really documented. What are exactly ghosts in the context of ASTLabelRegistry/ASTLabelRegistryBuilder?
2. I guess this might be already discuessed somewhere else, but wouldnt it be better to rename YulName to YulId/YulIdentifier? e.g. so that solidity::yul::Identifier is defined as struct Identifier { langutil::DebugData::ConstPtr debugData; YulId id; }; I think the type of YulName should only be YulName, if it can hold real names, and not just ids/indices/integers. This would improve readability of e.g. m_labels[_variableName.name] - I think name suggests too much that it’s a string, but it’s not.

[clonker]

Just read through this. Two things so far:

1. not strictly related to this PR, but I think its not really documented. What are exactly ghosts in the context of ASTLabelRegistry/ASTLabelRegistryBuilder?

Ghosts are documented here: https://github.com/ethereum/solidity/blob/develop/libyul/ASTLabelRegistry.h#L38-L41

2. I guess this might be already discuessed somewhere else, but wouldnt it be better to rename YulName to YulId/YulIdentifier? e.g. so that solidity::yul::Identifier is defined as struct Identifier { langutil::DebugData::ConstPtr debugData; YulId id; }; I think the type of YulName should only be YulName, if it can hold real names, and not just ids/indices/integers. This would improve readability of e.g. m_labels[_variableName.name] - I think name suggests too much that it’s a string, but it’s not.

It really should be YulLabelID or yul::LabelID, I guess. But IMO that should happen in a follow-up, the PR is already quite large :)

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