fix(astimport): accept sun/trx subdenominations in --import-ast

[yanghang8612]

Summary

--import-ast aborts on any contract that uses a TRON sun / trx literal, because the importer does not recognise those subdenominations.

The bug

ASTJsonExporter serialises the sun / trx subdenominations (via the generic TokenTraits::toString), but ASTJsonImporter::subdenomination() hardcodes only the upstream units (wei … years) and astAssert(false, "Unknown subdenomination") on anything else. So exporting and re-importing a contract with a TRX literal fails:

contract C {
    function f() public pure returns (uint) {
        return 1 trx;
    }
}

$ solc --import-ast export.json
Error: Failed to import AST: Unknown subdenomination

This breaks any AST round-trip tooling (--ast-compact-json → --import-ast) for TRON contracts using trx / sun units.

The fix

Add the sun and trx cases to ASTJsonImporter::subdenomination(), mapping to Literal::SubDenomination::Sun / Trx — the same enum values the exporter already emits — placed alongside ether to mirror the export ordering.

Targets release_0.8.28.

🤖 Generated with Claude Code

[github-actions]

Thank you for your contribution to the Solidity compiler! A team member will follow up shortly.

If you haven't read our contributing guidelines and our review checklist before, please do it now, this makes the reviewing process and accepting your contribution smoother.

If you have any questions or need our help, feel free to post them in the PR or talk to us directly on the #solidity-dev channel on Matrix.