This comes at the price of breaking compatibility and returning
"\\share" rather than just "share" from wxFileName::GetVolume() for the
UNC paths. This breakage seems justified because it is required in order
to allow application code to distinguish between paths "x:\foo" and
"\\x\foo", which was previously impossible as GetVolume() returned just
"x" in both cases.
Document this change, adjust the existing checks for the new GetVolume()
semantics and add a new test which passes now, but didn't pass before.
Closes#19255.
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