Sema: Properly track mangling number/name for linkage for using decls

Sema: Properly track mangling number/name for linkage for using decls

Using declarations which are aliases to struct types have their name
used as the struct type's name for linkage purposes.  Otherwise, make
sure to give an anonymous struct defined inside a using declaration a
mangling number to disambiguate it from other anonymous structs in the
same context.

This fixes PR22809.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231909 91177308-0d34-0410-b5e6-96231b3b80d8