[Modules TS] Module ownership semantics for redeclarations.

When declaring an entity in the "purview" of a module, it's never a
redeclaration of an entity in the purview of a default module or in no module
("in the global module"). Don't consider those other declarations as possible
redeclaration targets if they're not visible, and reject any cases where we
pick a prior visible declaration that violates this rule.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315251 91177308-0d34-0410-b5e6-96231b3b80d8