libstdc++: Use reserved form of [[__likely__]] in <variant>

We should not use [[unlikely]] before C++20, so use [[__unlikely__]]
instead.

libstdc++-v3/ChangeLog:

	* include/std/variant (_Variant_storage::_M_reset): Use
	__unlikely__ form of attribute instead of unlikely.

(cherry picked from commit 9f1cd51766f251aafe0f1b898892f79855892729)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 9abe7b9ed6d755fc1e6b43c8ae527d2244364140..75214c687dfd31ebb81707976a2462737101ddcf 100644 (file)
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -491,7 +491,7 @@ namespace __variant
       constexpr void
       _M_reset()
       {
-       if (!_M_valid()) [[unlikely]]
+       if (!_M_valid()) [[__unlikely__]]
          return;
 
        std::__do_visit<void>([](auto&& __this_mem) mutable