SyntaxProtocol.expand() does not expand peer macros introduced by other macros

### Description

This test fails due to `Foo` not having the second `bar` field (while `Bar` passes). This works correctly when expanded by the compiler plugin (although in this example it obviously result in a compilation error due to the property being redeclared).

```swift
public struct DuplicateAll: MemberAttributeMacro {
public static func expansion(
of node: AttributeSyntax,
attachedTo declaration: some DeclGroupSyntax,
providingAttributesFor member: some DeclSyntaxProtocol,
in context: some MacroExpansionContext
) throws -> [AttributeSyntax] {
return [AttributeSyntax(attributeName: SimpleTypeIdentifierSyntax(name: .identifier("Duplicate")))]
}
}

public struct Duplicate: PeerMacro {
public static func expansion(of node: AttributeSyntax, providingPeersOf declaration: some DeclSyntaxProtocol, in context: some MacroExpansionContext) throws -> [DeclSyntax] {
if var decl = declaration.as(VariableDeclSyntax.self) {
decl.attributes = nil
return [DeclSyntax(decl)]
}
return []
}
}

final class reproTests: XCTestCase {
func testMacro() {
assertMacroExpansion(
"""
@DuplicateAll
class Foo {
var bar: Int = 0
}
class Bar {
@Duplicate
var bar: Int = 0
}
""",
expandedSource: """

class Foo {
var bar: Int = 0
var bar: Int = 0
}
class Bar {
var bar: Int = 0
var bar: Int = 0
}
""",
macros: [
"DuplicateAll": DuplicateAll.self,
"Duplicate": Duplicate.self,
]
)
}
}
```

### Steps to Reproduce

_No response_