Implement P0599: 'noexcept for hash functions'. Fix a couple of hash functions (optional<T> and unique_ptr<T>) which were mistakenly marked as 'noexcept'. Reviewed as https://reviews.llvm.org/D31234

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@298573 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/thread b/include/thread
index 94c77e0..874ce3e 100644
--- a/include/thread
+++ b/include/thread
@@ -261,7 +261,7 @@
     : public unary_function<__thread_id, size_t>
 {
     _LIBCPP_INLINE_VISIBILITY
-    size_t operator()(__thread_id __v) const
+    size_t operator()(__thread_id __v) const _NOEXCEPT
     {
         return hash<__libcpp_thread_id>()(__v.__id_);
     }