aarch64: Fix parameter passing for [[no_unique_address]]

This patch makes the ABI code ignore zero-sized [[no_unique_address]]
fields when deciding whether something is a HFA or HVA.

As things stood, we'd get two sets of -Wpsabi warnings, one when
trying to decide whether something was an SVE function, and another
when actually processing the function definition or function call.
The patch therefore makes aapcs_vfp_sub_candidate honour the
CUMULATIVE_ARGS "silent_p" flag where applicable.

This doesn't stop all duplicate warnings for parameters, and I suspect
we'll get duplicate warnings for return values too, but it should be
better than nothing.

2020-04-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_function_arg_alignment): Add a
	comment explaining why we consider even zero-sized fields.
	(WARN_PSABI_EMPTY_CXX17_BASE): New constant.
	(WARN_PSABI_NO_UNIQUE_ADDRESS): Likewise.
	(aapcs_vfp_sub_candidate): Replace the boolean pointer parameter
	avoid_cxx17_empty_base with a pointer to a bitmask.  Ignore fields
	whose DECL_FIELD_ABI_IGNORED bit is set when determining whether
	something actually is a HFA or HVA.  Record whether we see a
	[[no_unique_address]] field that previous GCCs would not have
	ignored in this way.
	(aarch64_vfp_is_call_or_return_candidate): Add a parameter to say
	whether diagnostics should be suppressed.  Update the calls to
	aapcs_vfp_sub_candidate and report a -Wpsabi warning for the
	[[no_unique_address]] case.
	(aarch64_return_in_msb): Update call accordingly, never silencing
	diagnostics.
	(aarch64_function_value): Likewise.
	(aarch64_return_in_memory_1): Likewise.
	(aarch64_init_cumulative_args): Likewise.
	(aarch64_gimplify_va_arg_expr): Likewise.
	(aarch64_pass_by_reference_1): Take a CUMULATIVE_ARGS pointer and
	use it to decide whether arch64_vfp_is_call_or_return_candidate
	should be silent.
	(aarch64_pass_by_reference): Update calls accordingly.
	(aarch64_vfp_is_call_candidate): Use the CUMULATIVE_ARGS argument
	to decide whether arch64_vfp_is_call_or_return_candidate should be
	silent.

gcc/testsuite/
	* g++.target/aarch64/no_unique_address_1.C: New test.
	* g++.target/aarch64/no_unique_address_2.C: Likewise.