config.h: fix layering and don't duplicate definitions

Also correct the llvm-config.h header guard so it doesn't depend on 'CONFIG_H'
which is commonly defined in external projects and caused trouble for
embedders.

In future llvm/Config/llvm-config.h will be installed, but not
the private llvm/Config/config.h header.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210144 91177308-0d34-0410-b5e6-96231b3b80d8