c++: Drop TREE_READONLY on vars (possibly) initialized by tls wrapper [PR109164]

The following two testcases are miscompiled, because we keep TREE_READONLY
on the vars even when they are (possibly) dynamically initialized by a TLS
wrapper function.  Normally cp_finish_decl drops TREE_READONLY from vars
which need dynamic initialization, but for TLS we do this kind of
initialization upon every access to those variables.  Keeping them
TREE_READONLY means e.g. PRE can hoist loads from those before loops
which contain the TLS wrapper calls, so we can access the TLS variables
before they are initialized.

2023-03-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109164
	* cp-tree.h (var_needs_tls_wrapper): Declare.
	* decl2.cc (var_needs_tls_wrapper): No longer static.
	* decl.cc (cp_finish_decl): Clear TREE_READONLY on TLS variables
	for which a TLS wrapper will be needed.

	* g++.dg/tls/thread_local13.C: New test.
	* g++.dg/tls/thread_local13-aux.cc: New file.
	* g++.dg/tls/thread_local14.C: New test.
	* g++.dg/tls/thread_local14-aux.cc: New file.