[Threading] Move stack bounds fetching into the threading library.

Fetching the current stack bounds is done using threading functions, so
should be in the threading library.

rdar://90776105

lib/Threading/Linux.cpp

@@ -57,4 +57,23 @@ void swift::threading_impl::once_slow(once_t &predicate, void (*fn)(void *),
   linux::ulock_unlock(&predicate.lock);
 }
 
+swift::threading_impl::stack_bounds
+swift::threading_impl::thread_get_current_stack_bounds() {
+  stack_bounds result = { nullptr, nullptr };
+  pthread_attr_t attr;
+  size_t size = 0;
+  void *begin = nullptr;
+
+  if (!pthread_getattr_np(pthread_self(), &attr)) {
+    if (!pthread_attr_getstack(&attr, &begin, &size)) {
+      result.low = begin;
+      result.high = (char *)begin + size;
+    }
+
+    pthread_attr_destroy(&attr);
+  }
+
+  return result;
+}
+
 #endif // SWIFT_THREADING_LINUX