[cxx-interop] Workaround a compiler crash on Ubuntu 22.04

Swift is crashing on Ubuntu 22.04 when compiling benchmarks for `std::span`.

rdar://136875225

Author: egorzhdan

benchmark/cxx-source/CxxSpanTests.swift

@@ -11,16 +11,17 @@
 //===----------------------------------------------------------------------===//
 
 import TestsUtils
-import CxxStdlibPerformance
-import CxxStdlib // FIXME(rdar://128520766): this import should be redundant
-
-let iterRepeatFactor = 7
 
 // FIXME swift-ci linux tests do not support std::span
 #if os(Linux)
 public let benchmarks = [BenchmarkInfo]()
 #else
 
+import CxxStdlibPerformance
+import CxxStdlib // FIXME(rdar://128520766): this import should be redundant
+
+let iterRepeatFactor = 7
+
 public let benchmarks = [
   BenchmarkInfo(
     name: "CxxSpanTests.raw.iterator",