folly/wangle: temporarily disable compilation of Thens.cpp

Summary:
This code fails to compile with clang:dev, so don't try for now.
* folly/wangle/test/Thens.cpp: Don't attempt to compile test/Thens.cpp.
See 4412111 for details.  Prompted by clang:dev+MSAN effort, 4090011.

Test Plan:
Run this:
fbconfig --clang --with-project-version clang:dev -r folly/wangle
fbmake runtests
Failed before, passes with this patch.

Reviewed By: hans@fb.com

Subscribers: folly@lists, fugalh

FB internal diff: D1354751

Tasks: 4090011, 4412111

folly/wangle/test/Thens.cpp

@@ -2,6 +2,9 @@
 
 #include "Thens.h"
 
+#ifndef __clang__
+// TODO: fails to compile with clang:dev.  See task #4412111
+
 TEST(Future, thenVariants) {
   SomeClass anObject;
   Executor* anExecutor;
@@ -17,3 +20,5 @@ TEST(Future, thenVariants) {
   {Future<B> f = someFuture<A>().then(aStdFunction<B, Try<A>&&>());}
   {Future<B> f = someFuture<A>().then([&](Try<A>&&){return B();});}
 }
+
+#endif