[CIR][Builtin][NFC] More informative llvm_unreachable message in CIRGenBuiltinAArch64.cpp (#1124)

We are still seeing crash message like `NYI UNREACHABLE executed at
clang/lib/CIR/CodeGen/CIRGenBuiltinAArch64.cpp:3304`, which is not
convenient for triaging as our code base changes so fast, line number
doesn't help much.
So, here we replaced most of `llvm_unreachable("NYI")` with more
informative message.

Author: ghehg