[patch] libstdc++/70609 fix filesystem::copy()

[Jonathan Wakely]

The conditional code in filesystem::copy() that uses stdio_filebuf
didn't handle the case where the file to be copied is empty, and so
the ostream insertion doesn't do anything. I was also failing to check
for errors when closing the files.

Rather embarassingly, the copy.cc test was actually a duplicate of
absolute.cc and wasn't testing the copy function at all.

I also found some defects in the spec of filesystem::copy(), which
I've reported.

Tested x86_64-linux, committed to trunk.


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