GCC faithfully calculates visibility for variables independently of

GCC faithfully calculates visibility for variables independently of
whether it's a declaration or not, then ignores that information for
declarations unless it was explicitly given.  It's not totally clear
how that should be mapped into a sane system, but make an effort.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117780 91177308-0d34-0410-b5e6-96231b3b80d8