[OpenMP] Fix a wrong assertion in `__kmp_get_global_thread_id`

[tianshilei1992]

Summary

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.

[tianshilei1992]

This also fixes the issue introduced by D41167, which causes a test failure on Intel-based macOS.

[tianshilei1992]

make it cleaner

[jdoerfert]

Can we check if it's i or KMP_GTID_DNE instead?

[tianshilei1992]

fix comment

[tianshilei1992]

make it clear

openmp/runtime/src/kmp_runtime.cpp

for (i = 0; i < __kmp_threads_capacity; i++) {
     /* stack grows down -- search through all of the active threads */
 
     if (stack_addr <= stack_base) {
       size_t stack_diff = stack_base - stack_addr;
 
       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;
       }
     }
   }
 
   /* get specific to try and determine our gtid */
   KA_TRACE(1000,
            ("*** __kmp_get_global_thread_id: internal alg. failed to find "