[clang] Don't try to set MemberSpecialization on invalid var decl chain (PR #209604)

[Erich Keane via cfe-commits]

erichkeane wrote:

Build failures are unrelated to this patch (all LLDB and are on other things), but did a merge branch to hopefully get CI green.  

This IS a regression, so I'd love to hear your feelings on whether this should go on the release branch.  Crash-on-Invalid is one that I am much less motivated on, but it also seems pretty well self contained.  WDYT?

https://github.com/llvm/llvm-project/pull/209604