Revert r283029 - [cmake] Make LIT_COMMAND configurable and improve fallback support

Revert the change in r283029 (and the fixup in r283033) due to buildbot
breakage. The fixup is ineffective for the bots that do not force clean
build since the wrong value is already cached in CMakeCache.txt.

Reverting it should result in the cache variable being removed
and therefore it should be possible to re-introduce it after all
buildbots build this revision.

llvm-svn: 283036