[CIR][CodeGen][Builtin] `longjmp` handling

[tommymcm]

Minimized test case:

A() { __builtin_longjmp(A, 1); }

Failed assertion:

clangir/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp

Line 1847 in aeac352

llvm_unreachable("BI__builtin_longjmp NYI");

[yotto3s]

I want to try this for my first open-source contribution!

My understanding for this task is

• add definition for CIR_EhLongjmpOp in CIROps.td
• add class CIRToLLVMEhLongjmpOpLowering in LowerToLLVM.[h|cpp]
• implement the longjmp case in CIRGenBuiltin.cpp, almost the same as in clang/lib/CodeGen/CGBuiltin.cpp

[tommymcm]

That sounds like a good plan, you can reference the sister PR for more details #1895. Let me know if you have any issues or questions. It would also be nice to include some integration testing with EhSetjmpOp.