Outline string copy and clear code into helpers

[PavelKopyl]

NomicFoundation/solx-solidity#159

This includes:

1. Clearing fixed-size storage arrays with runtime loops

2. Outline string copy and clear code into helpers
   __sol.copy.<src>.storage.string for whole value copies to storage
   __sol.copy_string_data.storage.memory for the raw byte copy
   __sol.clear_string_tail for zeroing stale out-of-place data slots

Helpers are hosted by the enclosing contract as before. When there is
none (a free function that no contract references, lowered standalone
at module level), the module hosts them.

[Copilot]

Pull request overview

This PR factors previously inline string/bytes storage copy/cleanup logic into reusable sol.func helpers and expands helper hosting so helpers can be emitted at module scope when lowering standalone free functions.

Changes:

• Outline string tail-clearing, string-to-storage copy, and storage→memory string-data copy into dedicated helper functions (__sol.*) with inline backends.
• Extend getOrCreateHelperFn to host helpers either in the enclosing sol.contract or at module level when no contract is present.
• Replace some compile-time for loops used for storage clearing with BuilderExt::createCountedLoop (with optional unrolling).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
mlir/lib/Conversion/SolToYul/EVMUtil.cpp	Introduces helper-backed implementations for string copy/clear paths, adds new helper kinds/spellings, and allows module-level helper hosting.
mlir/include/mlir/Conversion/SolToYul/EVMUtil.h	Adds private inline-helper declarations and documents new helper emission behavior and helper kinds.

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[abinavpp]

mostly lgtm, just some nits. thank you!

Regarding the helper naming we discussed in DM: the only thing I could think of is moving the name generation to typed per-helper generators under a namespace, e.g.

namespace helpersym {
std::string copy(sol::DataLocation src, sol::DataLocation dst, StringRef tyName);
std::string clearStorage(sol::StructType ty);
std::string clearStringTail();
std::string copyStringData(sol::DataLocation src, sol::DataLocation dst);
}

getOrCreateHelperFn would take the resulting string as the key. This avoids the
string based approach and gives more structure to the naming. What do you think?

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[PavelKopyl]

mostly lgtm, just some nits. thank you!

Regarding the helper naming we discussed in DM: the only thing I could think of is moving the name generation to typed per-helper generators under a namespace, e.g.

namespace helpersym {
std::string copy(sol::DataLocation src, sol::DataLocation dst, StringRef tyName);
std::string clearStorage(sol::StructType ty);
std::string clearStringTail();
std::string copyStringData(sol::DataLocation src, sol::DataLocation dst);
}

getOrCreateHelperFn would take the resulting string as the key. This avoids the string based approach and gives more structure to the naming. What do you think?

Thank you, that makes sense, fixed.