This struct has some members that are accessed directly and others
that need accessors, but it's all just public. This is confusing, so
I've changed it to a class and made more members private.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218737 91177308-0d34-0410-b5e6-96231b3b80d8