Provide a hook during the sanitizer crash reporting for application-specific crash info

[chandlerc]

It would be really helpful for sanitizers to expose a hook for application-specific output during crash reporting.

For example, Clang and LLVM provide special pretty stack trace functionality and a link to their bug reporting URL in their crash reports, but these can't be provided along side sanitizer crashes for Clang and LLVM currently. The pretty stack trace functionality especially is useful when debugging sanitizer crashes of compilers.