[Sanitizers] Allow to install several internal Die callbacks.

This is required to properly re-apply r245770:
1) We should be able to dump coverage in __sanitizer::Die() if coverage
   collection is turned on.
2) We don't want to explicitly do this in every single
   sanitizer that supports it.
3) We don't want to link in coverage (and therefore symbolization) bits
   into small sanitizers that don't support it (safestack).

The solution is to make InitializeCoverage() register its own Die()
callback that would call __sanitizer_cov_dump(). This callback should be
executed in addition to another tool-specific die callbacks (if there
are any).

llvm-svn: 245889