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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 8cedf04a6c86d6858f6ea96d463819ae18cb76fa
diff --git a/src/condition_variable.cpp b/src/condition_variable.cpp
index e9b3739..b8426c6 100644
--- a/src/condition_variable.cpp
+++ b/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