[cxx-interop] Workaround a modularization issue in benchmarks

`import CxxStdlib` should not be required, since `CxxStdlibPerformance` transitively imports the C++ stdlib, but having the explicit import works around a modularization issue (rdar://128520766).

rdar://136330247

Author: egorzhdan

benchmark/cxx-source/CxxSpanTests.swift

@@ -12,6 +12,7 @@
 
 import TestsUtils
 import CxxStdlibPerformance
+import CxxStdlib // FIXME(rdar://128520766): this import should be redundant
 
 let iterRepeatFactor = 7