Adjust `llvm_unreachable` macro to account for platforms that don't define LLVM_BUILTIN_UNREACHABLE

[mehdi_amini]

Summary

Post 892c104fb7, LLVM_BUILTIN_UNREACHABLE may not be defined anymore.
Also when LLVM_UNREACHABLE_OPTIMIZE is OFF, emit LLVM_BUILTIN_UNREACHABLE
after LLVM_BUILTIN_TRAP to ensure that diagnostics are suppressed on
environments where LLVM_BUILTIN_TRAP is not marked as noreturn.

[dexonsmith]

LGTM!

llvm/include/llvm/Support/ErrorHandling.h

138–141

Maybe worth adding something about warnings?

Something like:

/// ... It conveys intent more clearly, suppresses diagnostics
/// for unreachable code paths, and
/// allows compilers to omit unnecessary code.

llvm/include/llvm/Support/ErrorHandling.h

 /// calling this function directly.
 [[noreturn]] void
 llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
                           unsigned line = 0);
 }
 
 /// 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, the behavior is controlled by the CMake flag
+/// 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 and
-/// allows compilers to omit some unnecessary code.
+/// Use this instead of assert(0). It conveys intent more clearly, suppresses
+/// diagnostics for unreachable code paths, and allows compilers to omit
+/// unnecessary code.
 #ifndef NDEBUG
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
-#elif !LLVM_UNREACHABLE_OPTIMIZE
-#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
-#elif defined(LLVM_BUILTIN_UNREACHABLE)
+#elif !defined(LLVM_BUILTIN_UNREACHABLE)
+#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#elif LLVM_UNREACHABLE_OPTIMIZE
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #else
-#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
 #endif
 
 #endif