[modules] When a declaration has non-trivial visibility, check whether it's

actually hidden before we check its linkage. This avoids computing the linkage
"too early" for an anonymous struct with a typedef name for linkage.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@253012 91177308-0d34-0410-b5e6-96231b3b80d8