[libcxx] Fix build bustage with threads disabled

Building with -DLIBCXX_ENABLE_THREADS=OFF -DLIBCXXABI_ENABLE_THREADS=OFF
(like e.g. for wasm) fails after D146228 because of a misplaced std
namespace begin/end.

Reviewed By: philnik, #libc

Differential Revision: https://reviews.llvm.org/D146682

Author: glandium

libcxx/include/__condition_variable/condition_variable.h

@@ -29,10 +29,10 @@
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#ifndef _LIBCPP_HAS_NO_THREADS
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+
 // enum class cv_status
 _LIBCPP_DECLARE_STRONG_ENUM(cv_status){no_timeout, timeout};
 _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(cv_status)
@@ -234,10 +234,10 @@ inline void condition_variable::__do_timed_wait(unique_lock<mutex>& __lk,
   wait_for(__lk, __tp - _Clock::now());
 }
 
-_LIBCPP_END_NAMESPACE_STD
-
 #endif // _LIBCPP_HAS_NO_THREADS
 
+_LIBCPP_END_NAMESPACE_STD
+
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___CONDITION_VARIABLE_CONDITION_VARIABLE_H