RFA (tree.h): C++ PATCH for c++/67407 (ICE with protected access)

[Jason Merrill]

In this testcase, the problem was that we were checking DERIVED_FROM_P, 
which uses dfs_walk_once and thus BINFO_MARKED, in the middle of a 
dfs_walk_once_accessible, which also uses BINFO_MARKED, and the marks 
from one walk were confusing the other walk.  Fixed by moving these 
binfo walking functions to use a hash_set instead of BINFO_MARKED.

After this change, there are no uses of BINFO_MARKED left in the source 
tree, so I'm inclined to rename it to BINFO_LANG_FLAG_0.  OK?

Tested x86_64-pc-linux-gnu.
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