[Bug libstdc++/83025] xfstream::open with char* gets caught on C++17 path overload.

[3dw4rd at verizon dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83025

--- Comment #1 from Ed Smith-Rowland <3dw4rd at verizon dot net> ---
Created attachment 42628
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42628&action=edit
patch...

enable_if with is_class_v.