This patch does 2 things:
- the default value of StackProtectorMode, StackProtectorMode::SSPOff now explicitly adds the nossp LLVM IR fn attr.
- it makes -fno-stack-protector/\GS_ now explictly pass -stack-protector 0 to cc1, which sets the StackProtectorMode to StackProtectorMode::SSPOff.
This allows safer mixing of translation units compiled with
-fno-stack-protector and -fstack-protector* for LTO, since LLVM IR fn
attr nossp will prevent inlining across the two.