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

For the tests, I wanted an -march setting that was stable enough
to use check-function-bodies and also wanted to force -mfloat-abi=hard.
I couldn't see any existing way of doing both together, since most
arm-related effective-target keywords are agnostic about the choice
between -mfloat-abi=softfp and -mfloat-abi=hard.  I therefore added
a new effective-target keyword for this combination.

I used the arm_arch_* framework for the effective-target rather than
writing a new set of custom Tcl routines.  This has the nice property
of separating the "compile and assemble" cases from the "link and run"
cases.  I only need compilation to work for the new tests, so requiring
linking to work would be an unnecessary restriction.

However, including an ABI requirement is arguably stretching what the
list was originally intended to handle.  The name arm_arch_v8a_hard
doesn't fit very naturally with some of the NEON-based tests.
On the other hand, the naming convention isn't entirely consistent,
so any choice would be inconsistent with something.

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

gcc/
	* doc/sourcebuild.texi (arm_arch_v8a_hard_ok): Document new
	effective-target keyword.
	(arm_arch_v8a_hard_multilib): Likewise.
	(arm_arch_v8a_hard): Document new dg-add-options keyword.
	* config/arm/arm.c (arm_return_in_memory): Note that the APCS
	code is deprecated and has not been updated to handle
	DECL_FIELD_ABI_IGNORED.
	(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.
	(aapcs_vfp_is_call_or_return_candidate): Update the calls to
	aapcs_vfp_sub_candidate and report a -Wpsabi warning for the
	[[no_unique_address]] case.  Use TYPE_MAIN_VARIANT in the
	diagnostic messages.
	(arm_needs_doubleword_align): Add a comment explaining why we
	consider even zero-sized fields.

gcc/testsuite/
	* lib/target-supports.exp: Add v8a_hard to the list of arm_arch_*
	targets.
	* g++.target/arm/no_unique_address_1.C: New test.
	* g++.target/arm/no_unique_address_2.C: Likewise.