`SyntaxProtocol.expand(macros:)` drops `IfConfigDeclSyntax` nodes from result

[grynspan]

Description

Given the following code:

let sourceCode = """
#if true
@inlinable
#endif
func f() {}
"""

let originalSyntax = Parser.parse(source: sourceCode)
print(originalSyntax)
let context = BasicMacroExpansionContext(lexicalContext: [], expansionDiscriminator: "", sourceFiles: [:])
let syntax = originalSyntax.expand(macros: [:]) { syntax in
  BasicMacroExpansionContext(sharingWith: context, lexicalContext: syntax.allMacroLexicalContexts())
}
print(syntax)

I would expect to see (ignoring potential reformatting):

#if true
@inlinable
#endif
func f() {}

But syntax drops everything in the IfConfigDeclSyntax node (#if onward) and just produces:

func f() {}

originalSyntax is correct, so the issue is during macro expansion, not the initial parse.

Steps to Reproduce

See above.

[ahoppen]

Synced to Apple’s issue tracker as rdar://141840779