[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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204346 91177308-0d34-0410-b5e6-96231b3b80d8