tree: Fix up warn_deprecated_use [PR104627]

The r12-7287-g1b71bc7c8b18bd1b change improved the -Wdeprecated
warning for C++, but regressed it for C, in particular in
gcc.dg/deprecated.c testcase we now report a type that actually isn't
deprecated as deprecated instead of the one that is deprecated.

The following change tries to find the middle ground between what
we used to do before and what r12-7287 change does.
If TYPE_STUB_DECL (node) is non-NULL (that is what happens with
those C tests), then it will do what it used to do before (just smarter,
there is no need to lookup_attribute when it is called again a few lines
below this), if it is NULL, it will try
TYPE_STUB_DECL (TYPE_MAIN_VARIANT (node)) - what the deprecated-16.C
test needs.

2022-03-04  Jakub Jelinek  <jakub@redhat.com>

	PR c/104627
	* tree.cc (warn_deprecated_use): For types prefer to use node
	and only use TYPE_MAIN_VARIANT (node) if TYPE_STUB_DECL (node) is
	NULL.