Don't redefine a bunch of defines from llvm-config.h in config.h.

r210144 made config.h include llvm-config.h and deduplicated defines. Then
rL239987 later added back some of the duplication.
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150914/300329.html
suggests this was done for the configure/make build, which no longer exists.

No intended behavior change.

https://reviews.llvm.org/D46288

llvm-svn: 331987