Wrap llvm_unreachable macro in do-while loop

Macros that expand into multiple terms can cause interesting preprocessor hickups depending
on the context they are used in. #56867 reported
a miscompilation of `llvm_unreachable(msg)` inside a `LLVM_DEBUG({ ... })` block. We were
able to fix it by wrapping the expansion in a `do {} while(false)`.

Differential Revision: https://reviews.llvm.org/D131337

Author: weliveindetail

llvm/include/llvm/Support/ErrorHandling.h

@@ -147,7 +147,11 @@ llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
 #elif LLVM_UNREACHABLE_OPTIMIZE
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #else
-#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
+#define llvm_unreachable(msg)                                                  \
+  do {                                                                         \
+    LLVM_BUILTIN_TRAP;                                                         \
+    LLVM_BUILTIN_UNREACHABLE;                                                  \
+  } while (false)
 #endif
 
 #endif