Improve the LLDB spec driver script

[HertzDevil]

• Uses bash instead of sh, since set -o is unavailable in the latter.
• Uses the debugger intrinsic instead of setting breakpoints manually in the LLDB script. This actually also ensures all the processes eventually terminate inside the debugger.
• Detects the LLVM version using the find-llvm-config script, required for FileCheck. Note that the LLDB and the LLVM versions need not agree, so the version suffix is not applied to the former.
• Adds a workaround for Segmentation fault during LibGC.init #11589 where GC initialization could emit a SIGSEGV intentionally.

[beta-ziliani]

LGTM, but the check for LLVM 14 is failing.