[C++] Make same_type_p return false for gnu_vector_type_p differences (PR 92789)

As Jason pointed out in the review of the C++ gnu_vector_type_p patch:

    https://gcc.gnu.org/ml/gcc-patches/2019-12/msg00173.html

the real fix for the XFAILs in acle/general-c++/gnu_vectors_*.C is to
make same_type_p return false for two types if one is gnu_vector_type_p
and the other isn't.  This patch does that and fixes the fallout.

Originally I'd tried to make it so that "X *" and "Y *" are
interconvertible whenever X and Y are, and similarly for
"X &" and "Y &".  That doesn't fall out naturally though,
and is different from how -flax-vector-conversions works.
The patch therefore accepts all the consequences of making
X and Y !same_type_p instead of trying to work around them.

2019-12-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/cp/
	PR c++/92789
	* typeck.c (structural_comptypes): Make sure that two vector types
	agree on gnu_vector_type_p.

gcc/testsuite/
	PR c++/92789
	* g++.dg/ext/sve-sizeless-2.C (statements): Expect pointer
	difference and comparisons between GNU and non-GNU types
	to be rejected.  Expect __is_same to be false for such pairs.
	* g++.target/aarch64/sve/acle/general-c++/gnu_vectors_1.C: Remove
	XFAILs.  Expect conversions between SVE vector pointers and
	GNU vector pointers to be rejected.  Test references.
	* g++.target/aarch64/sve/acle/general-c++/gnu_vectors_2.C: Likewise.

From-SVN: r279717