Fix-it Application Failure in `assertMacroExpansion` Function

[Matejkob]

Description

The fix-it provided by a peer macro is correctly applied by the compiler, yet the assertMacroExpansion function fails to apply it appropriately.

Steps to Reproduce

1. Reference the example Macro from SwiftSyntax repository: https://github.com/apple/swift-syntax/blob/098b177830c0d2e614b3757241b2572d758f9143/Examples/Sources/MacroExamples/Implementation/Peer/AddCompletionHandlerMacro.swift#L18-L164
2. Utilize the macro in the code as shown:

   struct Test {
     @AddCompletionHandler
     func fetchData() -> String {
       return "Hello, World!"
     }
   }

   The macro generates a diagnostic with a fix-it add 'async'.
3. When the fix-it is applied using the compiler, the async keyword is correctly inserted before -> String.
4. Construct a unit test to validate this scenario:

   func testExpansionOnNonAsyncFunctionAppliesFixIt() {
     assertMacroExpansion(
       """
       struct Test {
         @AddCompletionHandler
         func fetchData() -> String {
           return "Hello, World!"
         }
       }
       """,
       expandedSource: """
         struct Test {
           func fetchData() -> String {
             return "Hello, World!"
           }
         }
         """,
       diagnostics: [
         DiagnosticSpec(
           message: "can only add a completion-handler variant to an 'async' function",
           line: 3,
           column: 3,
           severity: .error,
           fixIts: [FixItSpec(message: "add 'async'")]
         )
       ],
       macros: macros,
       applyFixIts: ["add 'async'"],
       fixedSource: """
       struct Test {
         @AddCompletionHandler
         func fetchData() async -> String {
           return "Hello, World!"
         }
       }
       """,
       indentationWidth: .spaces(2)
     )
   }

   The expectation is for the test to pass, but it fails. The fixedSource string produced by the assertMacroExpansion function erroneously manifests as:

   struct Test {
     @AddCompletionHandler
     f() async -> String () -> String {
       return "Hello, World!"
     }
   }

   Notice the incorrect placement and duplication of the function signature, suggesting that the fix-it application is not handling the source code transformation as intended.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://118012820