Allow users to call ASan's deadly exception report mechanism

Users often have their own unhandled exception filters installed. ASan
already goes to great lengths to install its own filter, but our core
wars with Chrome crashpad have escalated to the point that its time to
declare a truce. By exposing this hook, they can call us directly when
they want ASan crash reporting without worrying about who initializes
when.

llvm-svn: 287040