c++: Support thread_local statics in header modules [PR113292]

Currently, thread_locals in header modules cause ICEs. This patch makes
the required changes for them to work successfully.

This requires additionally writing the DECL_TLS_MODEL for thread-local
variables to the module interface, and the TLS wrapper function needs to
have its DECL_BEFRIENDING_CLASSES written too as this is used to
retrieve what VAR_DECL it's a wrapper for when emitting a definition at
end of TU processing.

	PR c++/113292

gcc/cp/ChangeLog:
	* decl2.cc (get_tls_wrapper_fn): Set DECL_CONTEXT.
	(c_parse_final_cleanups): Suppress warning for no definition of
	TLS wrapper functions in header modules.
	* module.cc (trees_out::lang_decl_vals): Write wrapped variable
	for TLS wrapper functions.
	(trees_in::lang_decl_vals): Read it.
	(trees_out::decl_value): Write TLS model for thread-local vars.
	(trees_in::decl_value): Read it for new decls. Remember to emit
	definitions of TLS wrapper functions later.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr113292_a.H: New test.
	* g++.dg/modules/pr113292_b.C: New test.
	* g++.dg/modules/pr113292_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>