Make ~mutex and ~condition_variable trivial on Windows.

The implementations of __libcpp_mutex_destroy and __libcpp_condvar_destroy
are already NOPs, so this optimization is safe to perform.

See r365273 and PR27658 for more information.

llvm-svn: 365281
diff --git a/libcxx/src/condition_variable.cpp b/libcxx/src/condition_variable.cpp
index e9b3739..b8426c6 100644
--- a/libcxx/src/condition_variable.cpp
+++ b/libcxx/src/condition_variable.cpp
@@ -21,10 +21,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-condition_variable::~condition_variable()
-{
-    __libcpp_condvar_destroy(&__cv_);
-}
+// ~condition_variable is defined elsewhere.
 
 void
 condition_variable::notify_one() _NOEXCEPT