Add an atomic increment and decrement implementation, which will be used for

thread-safe reference counting.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73587 91177308-0d34-0410-b5e6-96231b3b80d8

configure

@@ -33760,6 +33760,8 @@ int main() {
         volatile unsigned long val = 1;
         __sync_synchronize();
         __sync_val_compare_and_swap(&val, 1, 0);
+        __sync_add_and_fetch(&val, 1);
+        __sync_sub_and_fetch(&val, 1);
         return 0;
       }