[AArch64] (PR94383) Avoid C++17 empty base field checking for HVA/HFA

[AArch64] (PR94383) Avoid C++17 empty base field checking for HVA/HFA

In C++17, an empty class deriving from an empty base is not an
aggregate, while in C++14 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
AArch64 PCS ABI and hence should not count towards determining whether an
object can be passed using the vector registers as per section
"6.4.2 Parameter Passing Rules" in the AArch64 PCS.
https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#the-base-procedure-call-standard

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 behaviour when run with `-std=gnu++14`.

> gcc -std=gnu++17 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:
    Bootstrap and regression test on aarch64-linux.
    All struct-layout-1 tests now pass.

gcc/ChangeLog:

2020-04-23  Matthew Malcomson  <matthew.malcomson@arm.com>
	    Jakub Jelinek  <jakub@redhat.com>

	PR target/94383
	* config/aarch64/aarch64.c (aapcs_vfp_sub_candidate): Account for C++17
	empty base class artificial fields.
	(aarch64_vfp_is_call_or_return_candidate): Warn when ABI PCS decision is
	different after this fix.