[msan] Cleanup public interface header.

Moved everything users are not supposed to use to a private interface header.
Documented all public interfaces. Made them safe to use even if built without
MemorySanitizer.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173800 91177308-0d34-0410-b5e6-96231b3b80d8