ASan: upon fiber switching, need to tell ASan

[JohanEngelen]

We need to call __sanitizer_start_switch_fiber and __sanitizer_finish_switch_fiber. Reading ASan's code, I think it's only needed when using FakeStack (stack-use-after-scope), but I'm not 100% sure.

See e.g. the calls in this code: https://github.com/facebook/folly/blob/master/folly/fibers/FiberManager.cpp

[JohanEngelen]

Reading ASan's code, I think it's only needed when using FakeStack (stack-use-after-scope), but I'm not 100% sure.

Nope, also needs to be done without fake stack such that ASan correctly reports about pointers lying inside a certain stack frame.

I've made some nice progress on this front. Main open question is how to deal with presence/absence of linked-in ASan runtime library.

[JohanEngelen]

#2975

[JohanEngelen]

Implemented by #2975