Global constant strings generated by nested macros cannot be written to binary files.

[Asura19]

Description

I use macro to create global constant string, it success.
But, when I use a nested macro to wrap #flat macro, it build success and expand as expected in XCode, but the constant string did not appears in mach-o file. (I used XMachOViewer to check executable binary file)

Reproduction

Attached is a reduced package that reproduces the behavior:
Test.zip

public struct FlatMacro: DeclarationMacro {
    
    public static func expansion(
        of node: some SwiftSyntax.FreestandingMacroExpansionSyntax,
        in context: some SwiftSyntaxMacros.MacroExpansionContext
    ) throws -> [DeclSyntax] {
        let argumentList = node.arguments
        var name: String = ""
        
        for argument in argumentList {
            switch argument.label?.text {
            case "name":
                if let stringLiteral = argument.expression.as(StringLiteralExprSyntax.self),
                   let hostValue = stringLiteral.segments.first?.as(StringSegmentSyntax.self)?.content.text {
                    name = hostValue
                }
            default:
                break
            }
        }
        
        let varName = "\(context.makeUniqueName("flat"))"
        let declarationString = """
            let \(varName) = "~~~~\(name)"
            """
        return [DeclSyntax(stringLiteral: declarationString)]
    }
}

public struct NestedMacro: DeclarationMacro {
    
    public static func expansion(
        of node: some SwiftSyntax.FreestandingMacroExpansionSyntax,
        in context: some SwiftSyntaxMacros.MacroExpansionContext
    ) throws -> [DeclSyntax] {
        return [
          """
          #flat(name: "456")
          """
        ]
    }
}

Expected behavior

The global constant "~~~~456" will be compiled to the mach-o binary file.

Environment

swift 6.0

Additional information

No response