Fix r359229 which tried to fix r359159...

When r359229 added noexcept to the declaration of `~mutex`, it didn't
add it to the definition which caused -Wimplicit-exception-spec-mismatch
to fire. This just adapts the definition to agree with the declaration.

llvm-svn: 359275

libcxx/src/mutex.cpp

@@ -19,7 +19,7 @@ const defer_lock_t  defer_lock = {};
 const try_to_lock_t try_to_lock = {};
 const adopt_lock_t  adopt_lock = {};
 
-mutex::~mutex()
+mutex::~mutex() _NOEXCEPT
 {
     __libcpp_mutex_destroy(&__m_);
 }