[Clang][ASan] Correct AsanDtorKindToString to return non-void in default case

Post D96572, a warning started showing up for me:
`clang/lib/Basic/Sanitizers.cpp:73:1: warning: control reaches end of non-void function [-Wreturn-type]`

So this adds a default to the case to return invalid, which seems appropriate,
and appears to correct the issue.

Differential Revision: https://reviews.llvm.org/D97496