Report proper error for named arguments in string/bytes.concat()

[tavian-dev]

Description

Fixes #15735.

Using named arguments with string.concat() or bytes.concat() previously triggered an internal compiler error (ICE) due to a solAssert(_functionCall.names().empty()) assertion. This replaces the assertions with proper TypeError diagnostics so the user gets a clear error message instead of a crash.

Reproduction (from the issue)

contract C {
    function f() public {
        string.concat({x: .3, y: "abc", z: true});
    }
}

Before: Internal compiler error: Solidity assertion failed at TypeChecker.cpp
After: TypeError: Named arguments cannot be used with string.concat().

The same fix is applied to bytes.concat() which had the identical assertion.

Changes

• libsolidity/analysis/TypeChecker.cpp: Replace solAssert with typeError + return in both typeCheckStringConcatFunction (error 4903) and typeCheckBytesConcatFunction (error 8145)
• Add syntax tests: string_concat_named_args.sol and bytes_concat_named_args.sol

Testing

• Built and ran syntax tests (3553 pass, including 2 new tests)
• Built and ran semantic tests (1636 pass)
• Verified the exact reproduction case from the issue produces a clean error instead of an ICE
• Code style check passes

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