We have two stacks that we set up during crash handling, both of
which were potentially misaligned. This only tripped us up on
Ubuntu 20.04 on aarch64 - my guess is that maybe `clone()` aligns
the stack pointer on newer kernels which is why we didn't see it
on 22.04.
rdar://110743884
These seem to be related to signal handling issues in threads that
have been started presumably by the C library or some other library.
Primarily this appears to affect Ubuntu 20.04 on aarch64, though I
have seen sporadic issues on 18.04 as well.
We address the problem by making the thread suspension mechanism
more robust in the face of threads with masked signals.
rdar://110653167
Mike and Max made various helpful suggestions, so I've added and updated
various comments and amended the code to cope with partial reads and
writes.
rdar://110261430
This also adds a function to demangle a symbol, and a way for the
backtracing code to report warning messages to the same place as
the main runtime.
I'd like to rename the _swift_isThunkFunction() SPI also, but we
can't do that until we've made the changes to the _Backtracing
library, so we'll do that there instead.
rdar://110261430
