There are bugs where you don't want the signal handler to trigger, most
notably when that will cause another crash. Examples of this are lldb
running out of memory or a bug in the reproducer generation code. This
adds an escape hatch trough a (developer oriented) flag to not install
the signal handler.
rdar://problem/65149595
Differential revision: https://reviews.llvm.org/D83496
10aa9e19fa