[LLDB/Reproducers] Add flag to avoid installing the signal handler.

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