cmake: Improve '-E create_symlink' edge case handling (#14713)
The logic added by commit ffc0b5e4 (Overwrite the symlink if it already exists, 2007-02-15) does not recognize and remove existing broken links before replacing them. Improve the logic to remove any existing destination file or link (but not directory). On failure, report an error message explaining why the existing path could not be removed or the new one could not be created. Add a RunCMake.CommandLine test to cover 'cmake -E' cases. Start with test cases covering 'cmake -E create_symlink' behavior on UNIX platforms.