[clang] [clang][ExprConst] Don't pass invalid decls to ASTRecordLayout (PR #203817)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

It looks like we do this pretty consistently through the code base but I wonder if we are missing this check in more places. 

https://github.com/llvm/llvm-project/pull/203817