Merge pull request #76755 from swiftlang/egorzhdan/benchmarks-modularization

[cxx-interop] Workaround a modularization issue in benchmarks

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