[split-file] Fix sys::fs::remove() on Solaris after D83834

where stdio.h ::remove() may set errno to EEXIST instead of ENOTEMPTY.

POSIX.1-2017 allows EEXIST for unlink() (which is called by remove()):

> [EEXIST] or [ENOTEMPTY]
> The flag parameter has the AT_REMOVEDIR bit set and the path argument names a directory that is not an empty directory, or there are hard links to the directory other than dot or a single entry in dot-dot.

Reviewed By: ro

Differential Revision: https://reviews.llvm.org/D85690