The _wrename function won't overwrite an existing file, so use
MoveFileEx instead. That allows renaming directories over files, which
POSIX doesn't allow, so check for that case explicitly and report an
error.
Also document the deviation from the expected behaviour, and add a test
for filesystem::rename which was previously missing.
The Filesystem TS experimental::filesystem::rename doesn't have that
extra code to handle directories correctly, so the relevant parts of the
new test are not run on Windows.
libstdc++-v3/ChangeLog:
* doc/xml/manual/status_cxx2014.xml: Document implementation
specific properties of std::experimental::filesystem::rename.
* doc/xml/manual/status_cxx2017.xml: Document implementation
specific properties of std::filesystem::rename.
* doc/html/*: Regenerate.
* src/c++17/fs_ops.cc (fs::rename): Implement correct behaviour
for directories on Windows.
* src/filesystem/ops-common.h (__gnu_posix::rename): Use
MoveFileExW on Windows.
* testsuite/27_io/filesystem/operations/rename.cc: New test.
* testsuite/experimental/filesystem/operations/rename.cc: New test.
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