[lldb] Add a "diagnostics dump" command

Add a "diagnostics dump" command to, as the name implies, dump the
diagnostics to disk. The goal of this command is to let the user
generate the diagnostics in case of an issue that doesn't cause the
debugger to crash.

This command is also critical for testing, where we don't want to cause
a crash to emit the diagnostics.

Differential revision: https://reviews.llvm.org/D135622