c++: Set DECL_CONTEXT for __cxa_thread_atexit [PR99187]

c++: Set DECL_CONTEXT for __cxa_thread_atexit [PR99187]

Modules streaming requires DECL_CONTEXT to be set on declarations that
are streamed. This ensures that __cxa_thread_atexit is given translation
unit context much like is already done with many other support
functions.

	PR c++/99187

gcc/cp/ChangeLog:

	* cp-tree.h (enum cp_tree_index): Add CPTI_THREAD_ATEXIT.
	(thread_atexit_node): New.
	* decl.cc (get_thread_atexit_node): Cache in thread_atexit_node.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99187.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Signed-off-by: Nathan Sidwell <nathan@acm.org>