[index] Index system ImportDecls even when there is a DeclarationsOnly filter

Whether we call an ImportDecl a decl or a reference symbol role is
somewhat academic, but in practice it's more like a declaration because
it is interesting even to consumers who wouldn't care about references.
Most importantly, we want to report the module dependencies of system
modules even when we have declaration-only filtering.

rdar://problem/27134855

llvm-svn: 275454