Refactored RegisterInfoInterface into its own header.

Also moved it into the lldb_private namespace.

The llgs branch is making use of this interface and its use is not
strictly limited to POSIX.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@209016 91177308-0d34-0410-b5e6-96231b3b80d8