Generalize sanitizer allocator public interface.

Introduce new public header <sanitizer/allocator_interface.h> and a set
of functions __sanitizer_get_ownership(), __sanitizer_malloc_hook() etc.
that will eventually replace their tool-specific equivalents
(__asan_get_ownership(), __msan_get_ownership() etc.). Tool-specific
functions are now deprecated and implemented as stubs redirecting
to __sanitizer_ versions (which are implemented differently in each tool).

Replace all uses of __xsan_ versions with __sanitizer_ versions in unit
and lit tests.

llvm-svn: 212469