[Docs] Update HowToSubmitABug

With the new PM imminent, bugpoint will diverge from opt, meaning it may
not reproduce a crash with the same arguments passed to opt. We need to
specify alternatives to bugpoint for reducing crashes.

I looked at the rest of the document to see if anything could be
improved. Major highlights:

* Run -Xclang -disable-llvm-passes instead of -O0 for skipping IR passes
* Mention the files that clang dumps on a crash
* Remove outdated reference to `delta` and plug `creduce` instead
* Mention llvm-reduce on top of bugpoint
* Mention --print-before-all --print-module-scope
* Mention sanitizers in addition to valgrind
* Mention opt-bisect for miscompiles

Reviewed By: fhahn, MaskRay

Differential Revision: https://reviews.llvm.org/D95578