Rename END_WITH_NULL to LLVM_END_WITH_NULL and move to Compiler.h

We shouldn't put this kind of attribute stuff in DataTypes.h.

Leave the END_WITH_NULL name for now so I can update clang without
making build spam.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221215 91177308-0d34-0410-b5e6-96231b3b80d8