[OpenMP] Fix a wrong assertion in __kmp_get_global_thread_id

The function assumes that `__kmp_gtid_get_specific` always returns a valid gtid.
That is not always true, because when creating the key for thread-specific data,
a destructor is assigned. The dtor will be called at thread exit. However, before
the dtor is called, the thread-specific data will be reset to NULL first
(https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_key_create.html):

> At thread exit, if a key value has a non-NULL destructor pointer, and the thread
> has a non-NULL value associated with that key, the value of the key is set to NULL.

This will lead to that `__kmp_gtid_get_specific` returns `KMP_GTID_DNE`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D159369

Author: shiltian

openmp/runtime/src/kmp_runtime.cpp

@@ -178,7 +178,12 @@ int __kmp_get_global_thread_id() {
       if (stack_diff <= stack_size) {
         /* The only way we can be closer than the allocated */
         /* stack size is if we are running on this thread. */
-        KMP_DEBUG_ASSERT(__kmp_gtid_get_specific() == i);
+        // __kmp_gtid_get_specific can return negative value because this
+        // function can be called by thread destructor. However, before the
+        // thread destructor is called, the value of the corresponding
+        // thread-specific data will be reset to NULL.
+        KMP_DEBUG_ASSERT(__kmp_gtid_get_specific() < 0 ||
+                         __kmp_gtid_get_specific() == i);
         return i;
       }
     }