CxxSpanReturnThunkBuilder: use _cxxOverrideLifetime(_:copying:)

Do not rely on the @_unsafeNonescapableResult attribute. That attribute is only
for temporarily working around bugs! And it only affects lifetime diagnostics within
the function. It has no affect on the caller's diagnostics, so it won't solve
this problem:

func macroGeneratedThunk() -> CxxSpan<Int> {
  return _unsafeRemoveLifetime(Span...)
}

We cannot simply add @_unsafeRemoveLifetime to the thunk, because SwiftSyntax
does not natively support the attribute. We don't want to add SwiftSyntax
support because this attribute will never be supported syntax!

Instead, use `_overrideLifetime` copying the `Void` type to remove a dependency:

func macroGeneratedThunk() -> CxxSpan<Int> {
  return _cxxOverrideLifetime(Span..., copying: ())
}

lib/Macros/Sources/SwiftMacros/SwiftifyImportMacro.swift

@@ -468,7 +468,7 @@ struct CxxSpanReturnThunkBuilder: BoundsCheckedThunkBuilder {
 
   func buildFunctionCall(_ pointerArgs: [Int: ExprSyntax]) throws -> ExprSyntax {
     let call = try base.buildFunctionCall(pointerArgs)
-    return "_unsafeRemoveLifetime(Span(_unsafeCxxSpan: \(call)))"
+    return "_cxxOverrideLifetime(Span(_unsafeCxxSpan: \(call)), copying: ())"
   }
 }