aarch64: Fix arm_sve_vector_bits on typedefs [PR95105]

aarch64: Fix arm_sve_vector_bits on typedefs [PR95105]

Compiling this testcase with -march=armv8.2-a+sve
-msve-vector-bits=512:

----------------------------------------------------------
typedef __SVFloat32_t foo;
typedef foo bar __attribute__((arm_sve_vector_bits(512)));
template<typename T> struct s { T x; };
extern s<bar> a;
bar &b = a.x;
----------------------------------------------------------

gave the bogus error:

  cannot bind non-const lvalue reference of type ‘bar&’ to an rvalue
  of type ‘bar’

The testcase works if the attribute is applied directly
to __SVFloat32_t instead of via foo.

This shows a more general problem with the way that we were handling
the arm_sve_vector_bits attribute: we started by building a distinct
copy of the type to which the attribute was applied, instead of starting
with its main variant.  This new type then became its own main variant,
meaning that the relationship between types that have the attribute
could be different from the relationship between types that don't have
the attribute.

This patch instead copies the main variant of the original type and then
reapplies all the differences.

2020-05-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR target/95105
	* config/aarch64/aarch64-sve-builtins.cc
	(handle_arm_sve_vector_bits_attribute): Create a copy of the
	original type's TYPE_MAIN_VARIANT, then reapply all the differences
	between the original type and its main variant.

gcc/testsuite/
	PR target/95105
	* gcc.target/aarch64/sve/acle/general/attributes_8.c: New test.
	* g++.target/aarch64/sve/acle/general-c++/attributes_1.C: Likewise.