[MSan] add KMSAN support to Clang driver

Boilerplate code for using KMSAN instrumentation in Clang.

We add a new command line flag, -fsanitize=kernel-memory, with a
corresponding SanitizerKind::KernelMemory, which, along with
SanitizerKind::Memory, maps to the memory_sanitizer feature.

KMSAN is only supported on x86_64 Linux.

It's incompatible with other sanitizers, but supports code coverage
instrumentation.

llvm-svn: 341641