[Arm] Account for C++17 artificial field determining Homogeneous Aggregates

In C++14, an empty class deriving from an empty base is not an
aggregate, while in C++17 it is.  In order to implement this, GCC adds
an artificial field to such classes.

This artificial field has no mapping to Fundamental Data Types in the
Arm PCS ABI and hence should not count towards determining whether an
object can be passed using the vector registers as per section
"7.1.2 Procedure Calling" in the arm PCS
https://developer.arm.com/docs/ihi0042/latest?_ga=2.60211309.1506853196.1533541889-405231439.1528186050

This patch avoids counting this artificial field in
aapcs_vfp_sub_candidate, and hence calculates whether such objects
should be passed in vector registers in the same manner as C++14 (where
the artificial field does not exist).

Before this change, the test below would pass the arguments to `f` in
general registers.  After this change, the test passes the arguments to
`f` using the vector registers.

The new behaviour matches the behaviour of `armclang`, and also matches
the GCC behaviour when run with `-std=gnu++14`.

> gcc -std=gnu++17 -march=armv8-a+simd -mfloat-abi=hard test.cpp

``` test.cpp
struct base {};

struct pair : base
{
  float first;
  float second;
  pair (float f, float s) : first(f), second(s) {}
};

void f (pair);
int main()
{
  f({3.14, 666});
  return 1;
}
```

We add a `-Wpsabi` warning to catch cases where this fix has changed the ABI for
some functions.  Unfortunately this warning is not emitted twice for multiple
calls to the same function, but I feel this is not much of a problem and can be
fixed later if needs be.

(i.e. if `main` called `f` twice in a row we only emit a diagnostic for the
first).

Testing:
    Bootstrapped and regression tested on arm-linux.
    This change fixes the struct-layout-1 tests Jakub added
    https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544204.html
    Regression tested on arm-none-eabi.

gcc/ChangeLog:

2020-04-28  Matthew Malcomson  <matthew.malcomson@arm.com>
	    Jakub Jelinek  <jakub@redhat.com>

	PR target/94711
	* config/arm/arm.c (aapcs_vfp_sub_candidate): Account for C++17 empty
	base class artificial fields.
	(aapcs_vfp_is_call_or_return_candidate): Warn when PCS ABI
	decision is different after this fix.

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