[Sanitizers] UBSan unreachable incompatible with Kernel ASan

Summary:
This is a follow up for https://reviews.llvm.org/D57278. The previous
revision should have also included Kernel ASan.

rdar://problem/40723397

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57711

llvm-svn: 353120