[MS ABI] Number unnamed TagDecls which aren't externally visible

TagDecls (structs, enums, etc.) may have the same name for linkage
purposes of one another; to disambiguate, we add a number to the mangled
named.  However, we didn't do this if the TagDecl has a pseudo-name for
linkage purposes (it was defined alongside a DeclaratorDecl or a
TypeNameDecl).

This fixes PR24651.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246659 91177308-0d34-0410-b5e6-96231b3b80d8