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

This was added to a later PR, but not to this one, though we need
it here.

rdar://110261430

This should have been disabled until apple#66338.

rdar://110261430

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

Moved the comment for `_swift_backtrace_demangle` into the header file
instead of it being in the implementation.

rdar://110261430