c++: Fix -Weffc++ in templates [PR98841]

c++: Fix -Weffc++ in templates [PR98841]

We emit a bogus warning on the following testcase, suggesting that the
operator should return *this even when it does that already.
The problem is that normally cp_build_indirect_ref_1 ensures that *this
is folded as current_class_ref, but in templates (if return type is
non-dependent, otherwise check_return_expr doesn't check it) it didn't
go through cp_build_indirect_ref_1, but just built another INDIRECT_REF.
Which means it then doesn't compare pointer-equal to current_class_ref.

The following patch fixes it by doing in build_x_indirect_ref for
*this what cp_build_indirect_ref_1 would do.

2021-01-28  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98841
	* typeck.c (build_x_indirect_ref): For *this, return current_class_ref.

	* g++.dg/warn/effc5.C: New test.