This ports the change from TSan (llvm@0784b1e).

A key difference is that TSan initializes the allocator prior to
CheckAndProtect, while MSan initializes the allocator afterwards;
this slightly simplifies the MSan patch. Nonetheless, we need to check
that the allocator layout is compatible with ASLR. Since the information is
not readily available in msan.h, we duplicate the information from
msan_allocator.cpp, and create a new MappingDesc::ALLOCATOR type.

Testing notes: run 'sudo sysctl vm.mmap_rnd_bits=32; ninja check-msan'
before and after this patch.

… we must

still return false. (The second InitShadow call does not perform the
same checks that the first InitShadow call did.)