[cmake] Move LLDB_DISABLE_LIBEDIT handling code into a central place

This was previously scattered between the main CMakeLists.txt file and
LLDBGenerateConfig.cmake and LLDBConfig.cmake. This caused the some of
the code to be executed in incorrect order. Specifically, the check for
el_winsertstr was done before libedit_LIBRARIES was computed, and so it
always failed on the first run.

Moving it the two checks to a central place makes sure this doesn't
happen again and improves the overall readability.

llvm-svn: 355103