Add an option to disable libedit

Summary: This should provide the function similar to `--disable-libedit` with the autotools build system, which seems to be missing from the commit (r200595) that adds this.

Reviewers: pcc, beanz

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26550

llvm-svn: 287293