[cxx-interop] Re-enable benchmarks

This adds an `import CxxStdlib` statement which fixes compilation. It should be redundant, but it works around a bug that got exposed by an change in explicit modules (rdar://128520766).

This will bring back the performance numbers while the underlying issue is being investigated.

Author: egorzhdan

benchmark/CMakeLists.txt

@@ -209,11 +209,9 @@ set(SWIFT_BENCH_MODULES
     single-source/WordCount
     single-source/XorLoop
     cxx-source/CreateObjects
-    # Disabled for rdar://128520766
-    # cxx-source/CxxSetToCollection
+    cxx-source/CxxSetToCollection
     cxx-source/CxxStringConversion
-    # Disabled for rdar://128520766
-    # cxx-source/CxxVectorSum
+    cxx-source/CxxVectorSum
     # TODO: rdar://92120528
     # cxx-source/ReadAccessor
 )

benchmark/cxx-source/CxxSetToCollection.swift

@@ -24,6 +24,7 @@ public let benchmarks = [BenchmarkInfo]()
 
 import CxxStdlibPerformance
 import Cxx
+import CxxStdlib // FIXME(rdar://128520766): this import should be redundant
 
 public let benchmarks = [
   BenchmarkInfo(

benchmark/cxx-source/CxxVectorSum.swift

@@ -24,6 +24,7 @@ public let benchmarks = [BenchmarkInfo]()
 
 import CxxStdlibPerformance
 import Cxx
+import CxxStdlib // FIXME(rdar://128520766): this import should be redundant
 
 public let benchmarks = [
   BenchmarkInfo(