Host: unconstify editline paramters

Although the interface to el_push should be a constant parameter (as it is on
Darwin), certain Linux distributions currently ship a header which does not
provide proper const correctness.  This causes compilation failures on Linux.
Strip the constness on the parameter, which whilst incorrect, is mostly
harmless.  The parameter will not be changed by the interface and so it is
acceptable to do this.  When distributions have updated to a more correct
declaration, it would be nice to revert this change.

Addresses PR18784.

llvm-svn: 201092