[msan] Change track-origins default mode from 1 to 2.

Change -fsanitize-memory-track-origins to be equivalent to
-fsanitize-memory-track-origins=2.

Track-origins=2 provides a lot more detailed reports at the cost of
some additional slowdown (ranging from none to, sometimes, 3x; ~3% average on
SPEC2006).

llvm-svn: 230644