Cache visibility of decls.

This unifies the linkage and visibility caching. I first implemented this when
working on pr13844, but the previous fixes removed the performance advantage of
this one.

This is still a step in the right direction for making linkage and visibility
cheap to use.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171048 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed