As noticed in D87637, when LLDB crashes, we only print stack traces if
LLDB is directly executed, not when used via Python bindings. Enabling
this by default may be undesirable (libraries shouldn't be messing with
signal handlers), so make this an explicit opt-in.
I "commandeered" this patch from Jordan Rupprecht who put this up for
review originally.
Differential revision: https://reviews.llvm.org/D91835
