PR libstdc++/90397 fix std::variant friend declarations

Clang diagnoses the inconsistent noexcept-specifier on the friend
declaration of __get. Add it, and also on __get_storage.

	PR libstdc++/90397
	* include/std/variant (_Variant_storage<false, Types...>::_M_storage())
	(_Variant_storage<true, Types...>::_M_reset()))
	(_Variant_storage<true, Types...>::_M_storage())): Add noexcept.
	(__get_storage): Likewise.
	(variant): Add noexcept to friend declarations for __get and
	__get_storage.

From-SVN: r271079

libstdc++-v3/ChangeLog

 2019-05-10  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/90397
+	* include/std/variant (_Variant_storage<false, Types...>::_M_storage())
+	(_Variant_storage<true, Types...>::_M_reset()))
+	(_Variant_storage<true, Types...>::_M_storage())): Add noexcept.
+	(__get_storage): Likewise.
+	(variant): Add noexcept to friend declarations for __get and
+	__get_storage.
+
 	PR libstdc++/90388
 	* include/bits/unique_ptr.h (default_delete, default_delete<T[]>):
 	Use _Require for constraints.

libstdc++-v3/include/std/variant

@@ -402,7 +402,7 @@ namespace __variant
       { _M_reset(); }
 
       void*
-      _M_storage() const
+      _M_storage() const noexcept
       {
 	return const_cast<void*>(static_cast<const void*>(
 	    std::addressof(_M_u)));
@@ -432,11 +432,11 @@ namespace __variant
 	_M_index(_Np)
 	{ }
 
-      void _M_reset()
+      void _M_reset() noexcept
       { _M_index = variant_npos; }
 
       void*
-      _M_storage() const
+      _M_storage() const noexcept
       {
 	return const_cast<void*>(static_cast<const void*>(
 	    std::addressof(_M_u)));
@@ -760,7 +760,7 @@ namespace __variant
 
   // Returns the raw storage for __v.
   template<typename _Variant>
-    void* __get_storage(_Variant&& __v)
+    void* __get_storage(_Variant&& __v) noexcept
     { return __v._M_storage(); }
 
   template <typename _Maybe_variant_cookie, typename _Variant>
@@ -1556,10 +1556,12 @@ namespace __variant
 #endif
 
       template<size_t _Np, typename _Vp>
-	friend constexpr decltype(auto) __detail::__variant::__get(_Vp&& __v);
+	friend constexpr decltype(auto)
+	__detail::__variant::__get(_Vp&& __v) noexcept;
 
       template<typename _Vp>
-	friend void* __detail::__variant::__get_storage(_Vp&& __v);
+	friend void*
+	__detail::__variant::__get_storage(_Vp&& __v) noexcept;
 
 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP) \
       template<typename... _Tp> \