[msan] Handle mixed track-origins and keep-going settings (llvm part).

Before this change, each module defined a weak_odr global __msan_track_origins
with a value of 1 if origin tracking is enabled, 0 if disabled. If there are
modules with different values, any of them may win. If 0 wins, and there is at
least one module with 1, the program will most likely crash.

With this change, __msan_track_origins is only emitted if origin tracking is
on. Then runtime library detects if there is at least one module with origin
tracking, and enables runtime support for it.