[lsan] Introduce __lsan_(un)register_root_region().

Add an interface for telling LSan that a region of memory is to be treated as a
source of live pointers. Useful for code which stores pointers in mapped memory.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@197489 91177308-0d34-0410-b5e6-96231b3b80d8