Don't consider visibility from template parameter lists if we're

Don't consider visibility from template parameter lists if we're
computing for a nested decl with explicit visibility.  This is all part
of the general philosophy of explicit visibility attributes, where
any information that was obviously available at the attribute site
should probably be ignored.  Fixes PR9371.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126992 91177308-0d34-0410-b5e6-96231b3b80d8