[index] Add indexing for unresolved-using declarations

[index] Add indexing for unresolved-using declarations

In dependent contexts we end up referencing these, so make sure they
have USRs, and have their declarations indexed. For the most part they
behave like typedefs, but we also need to worry about having multiple
using declarations with the same "name".

rdar://problem/33883650

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311053 91177308-0d34-0410-b5e6-96231b3b80d8