[PR88146] avoid diagnostics diffs if cdtor_returns_this

[PR88146] avoid diagnostics diffs if cdtor_returns_this

Diagnostics for testsuite/g++.dg/cpp0x/inh-ctor32.C varied across
platforms.  Specifically, on ARM, the diagnostics within the subtest
derived_ctor::inherited_derived_ctor::constexpr_noninherited_ctor did
not match those displayed on other platforms, and the test failed.

The difference seemed to have to do with locations assigned to ctors,
but it was more subtle: on ARM, the instantiation of bor's template
ctor was nested within the instantiation of bar's template ctor
inherited from bor.  The reason turned out to be related with the
internal return type of ctors: arm_cxx_cdtor_returns_this is enabled
for because of AAPCS, while cxx.cdtor_returns_this is disabled on most
other platforms.  While convert_to_void returns early with a VOID
expr, the non-VOID return type of the base ctor CALL_EXPR causes
convert_to_void to inspect the called decl for nodiscard attributes:
maybe_warn_nodiscard -> cp_get_fndecl_from_callee ->
maybe_constant_init -> cxx_eval_outermost_constant_expr ->
instantiate_constexpr_fns -> nested instantiation.

The internal return type assigned to a cdtor should not affect
instantiation (constexpr or template) decisions, IMHO.  We know it
affects diagnostics, but I have a hunch this might bring deeper issues
with it, so I've arranged for the CALL_EXPR handler in convert_to_void
to disregard cdtors, regardless of the ABI.

for  gcc/cp/ChangeLog

	PR c++/88146
	* cvt.c (convert_to_void): Handle all cdtor calls as if
	returning void.

From-SVN: r268004