Merge commit 81b20e110b3f from llvm git (by Roland McGrath):

[jhb]

Summary

[libc++] Work around new GCC 15 type_traits builtins that can't be
used as Clang's can (#137871)

GCC 15 has added builtins for various C++ type traits that Clang
already had.  Since `__has_builtin(...)` now finds these, the #if
branches previously only used for Clang are now used for GCC 15.
However, GCC 15 requires that these builtins only be used in type
aliases, not in template aliases.

For now, just don't use the `__has_builtin(...)` branches under newer
GCC versions, so both 14 and 15 work during the transition.  This
can be cleaned up later to use all the GCC 15 builtins available.

Fixed: #137704
Fixed: #117319

[dim]

LGTM (this will give a minor conflict in my llvm-21-update branch, but I'll fix that there :)