[MSan] Add KMSAN instrumentation to MSan pass

Introduce the -msan-kernel flag, which enables the kernel instrumentation.

The main differences between KMSAN and MSan instrumentations are:

- KMSAN implies msan-track-origins=2, msan-keep-going=true;
- there're no explicit accesses to shadow and origin memory.
  Shadow and origin values for a particular X-byte memory location are
  read and written via pointers returned by
  __msan_metadata_ptr_for_load_X(u8 *addr) and
  __msan_store_shadow_origin_X(u8 *addr, uptr shadow, uptr origin);
- TLS variables are stored in a single struct in per-task storage. A call
  to a function returning that struct is inserted into every instrumented
  function before the entry block;
- __msan_warning() takes a 32-bit origin parameter;
- local variables are poisoned with __msan_poison_alloca() upon function
  entry and unpoisoned with __msan_unpoison_alloca() before leaving the
  function;
- the pass doesn't declare any global variables or add global constructors
  to the translation unit.

llvm-svn: 341637