Fix for_each and fetch unit-tests under MSVC

Summary: [Folly] Refactor `for_each` and `fetch` to fix their unit-tests under MSVC. New code is smaller.

Reviewed By: Orvid

Differential Revision: D9569412

fbshipit-source-id: 87807ddf41f1dbd16244a82c5be30eae08dbdf4c

CMakeLists.txt

@@ -427,8 +427,7 @@ if (BUILD_TESTS)
       TEST f14_fwd_test SOURCES F14FwdTest.cpp
       TEST f14_map_test SOURCES F14MapTest.cpp
       TEST f14_set_test SOURCES F14SetTest.cpp
-      TEST foreach_test WINDOWS_DISABLED
-        SOURCES ForeachTest.cpp
+      TEST foreach_test SOURCES ForeachTest.cpp
       TEST merge_test SOURCES MergeTest.cpp
       TEST sparse_byte_set_test SOURCES SparseByteSetTest.cpp