[cmake] Modify libedit target name to match LLVM's FindLibEdit
For unified builds, LLVM with the update to stable/20221013 includes its
own FindLibEdit, which shares many of the same names than Swift's
FindLibEdit, except that they differ in the target name.
Match both LLVM and Swift target names, so unified builds can find
LibEdit::LibEdit correctly and not try to link against libedit
mistakenly.
This should not affect Darwin/Linux builds from upstream Swift, since
they build non-unified in most people setups (including CI).
