Communicate to llvm/gcc that refcount and index errors are very unlik…

…ely.

Make refcount errors fatal.

src/neat/runtime/array.nt

@@ -1,12 +1,11 @@
 module neat.runtime.array;
 
+extern(C) void neat_runtime_index_oob(size_t index);
+
 size_t checkIndex(size_t index, size_t length) {
     // < 0 overflows to >= length.
     if (index >= length) {
-        import neat.runtime.stdlib : exit, fprintf, stderr;
-
-        fprintf(stderr, "Array index out of bounds: %zd\n".ptr, index);
-        exit(1);
+        neat_runtime_index_oob(index);
     }
     return index;
 }

src/runtime.c

@@ -172,10 +172,19 @@ void print_backtrace()
 #endif
 }
 
-void neat_runtime_refcount_violation(struct String s, ptrdiff_t *ptr)
+#define FATALERROR __attribute__((cold)) __attribute__((noinline)) __attribute__((noreturn))
+
+void FATALERROR neat_runtime_refcount_violation(struct String s, ptrdiff_t *ptr)
 {
     printf("<%.*s: refcount logic violated: %zd at %p\n", (int) s.length, s.ptr, *ptr, ptr);
     print_backtrace();
+    exit(1);
+}
+
+void FATALERROR neat_runtime_index_oob(size_t index)
+{
+    fprintf(stderr, "Array index out of bounds: %zd\n", index);
+    exit(1);
 }
 
 void neat_runtime_refcount_inc(struct String s, ptrdiff_t *ptr)