[MSan] Add __msan_unpoison_string() to the public interface.

Using __msan_unpoison() on null-terminated strings is awkward because
strlen() can't be called on a poisoned string. This case warrants a special
interface function.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204448 91177308-0d34-0410-b5e6-96231b3b80d8