[msan] -fsanitize-memory-track-origins=[level] flag and docs.

This change turns -fsanitize-memory-track-origins into
-fsanitize-memory-track-origins=[level] flag (keeping the old one for
compatibility). Possible levels are 0 (off), 1 (default) and 2 (incredibly
detailed). See docs (part of this patch) for more info.

llvm-svn: 204346