[bazel] Set the default of LLVM_UNREACHABLE_OPTIMIZE to 1

This matches the CMake default. LLVM_UNREACHABLE_OPTIMIZE=0 doesn't
build with gcc

Author: d0k

utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/llvm-config.h

@@ -122,6 +122,6 @@
 
 /* Define if llvm_unreachable should be optimized with undefined behavior
  * in non assert builds */
-/* #undef LLVM_UNREACHABLE_OPTIMIZE */
+#define LLVM_UNREACHABLE_OPTIMIZE 1
 
 #endif