Replace the interim lli build fix with something cleaner

Eliminates the LLI_BUILDING_CHILD build hack from r199885.

Also add a FIXME to remove code that tricks the tests into passing when the
feature fails to work. Please don't do stuff like this, the tests exist for a
reason!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199929 91177308-0d34-0410-b5e6-96231b3b80d8