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.
Merge topic 'cmake-E-symlink-replace-dangling' into next

e0228e2b cmake: Improve '-E create_symlink' edge case handling (#14713)