一、llvm_unreachable
用来描述某个不支持的特性，会比assert(0) 更优雅，dump出来的信息也会友好一些
官方注释，llvm-project/llvm/include/llvm/Support/ErrorHandling.h:125
/// Marks that the current location is not supposed to be reachable.
/// In !NDEBUG builds, prints the message and location info to stderr.
/// In NDEBUG builds, if the platform does not support a builtin unreachable
/// then we call an internal LLVM runtime function. Otherwise the behavior is
/// controlled by the CMake flag
/// -DLLVM_UNREACHABLE_OPTIMIZE
/// * When "ON" (default) llvm_unreachable() becomes an optimizer hint
/// that the current location is not supposed to be reachable: the hint
/// turns such code path into undefined behavior. On compilers that don't
/// support such hints, prints a reduced message instead and aborts the
/// program.
/// * When "OFF", a builtin_trap is emitted instead of an
// optimizer hint or printing a reduced message.
///
/// Use this instead of assert(0). It conveys intent more clearly, suppresses
/// diagnostics for unreachable code paths, and allows compilers to omit
/// unnecessary code.
二、 LLVM_DEBUG
LLVM Debug信息输出，需要使用-g选项的Debug版本，可以使用--debug输出所有LLVM_DEBUG内容，也可以使用--debug-only控制输出某个DEBUG_TYPE的输出
三、dump
基本所有的LLVM数据都有dump方法，是一个调试非常好用的东西