rs6000: Fix C++14 vs. C++17 ABI bug on powerpc64le [PR94707]

rs6000: Fix C++14 vs. C++17 ABI bug on powerpc64le [PR94707]

As mentioned in the PR and on IRC, the recently added struct-layout-1.exp
new tests FAIL on powerpc64le-linux (among other targets).
FAIL: tmpdir-g++.dg-struct-layout-1/t032 cp_compat_x_tst.o-cp_compat_y_tst.o execute
FAIL: tmpdir-g++.dg-struct-layout-1/t058 cp_compat_x_tst.o-cp_compat_y_tst.o execute
FAIL: tmpdir-g++.dg-struct-layout-1/t059 cp_compat_x_tst.o-cp_compat_y_tst.o execute
in particular.  The problem is that the presence or absence of the C++17
artificial empty base fields, which have non-zero TYPE_SIZE, but zero
DECL_SIZE, change the ABI decisions, if it is present (-std=c++17), the type
might not be considered homogeneous, while if it is absent (-std=c++14), it
can be.

The following patch fixes that and emits a -Wpsabi inform; perhaps more
often than it could, because the fact that rs6000_discover_homogeneous_aggregate
returns true when it didn't in in GCC 7/8/9 with -std=c++17 doesn't still
mean it will make a different ABI decision, but the warning triggered only
on the test I've changed (the struct-layout-1.exp tests use -w -Wno-psabi
already).

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR target/94707
	* config/rs6000/rs6000-call.c (rs6000_aggregate_candidate): Add
	cxx17_empty_base_seen argument.  Pass it to recursive calls.
	Ignore cxx17_empty_base_field_p fields after setting
	*cxx17_empty_base_seen to true.
	(rs6000_discover_homogeneous_aggregate): Adjust
	rs6000_aggregate_candidate caller.  With -Wpsabi, diagnose homogeneous
	aggregates with C++17 empty base fields.

	* g++.dg/tree-ssa/pr27830.C: Use -Wpsabi -w for -std=c++17 and higher.