[modules] A using-declaration doesn't introduce a new entity, just a new name

[modules] A using-declaration doesn't introduce a new entity, just a new name
for an existing entity, and as such a using-declaration doesn't need to
conflict with a hidden entity (nor vice versa).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247654 91177308-0d34-0410-b5e6-96231b3b80d8