aarch64: Tweaks to the handling of fixed-length SVE types

aarch64: Tweaks to the handling of fixed-length SVE types

This patch is really four things rolled into one, since separating
them seemed artificial:

- Update the mangling of the fixed-length SVE ACLE types to match
  the upcoming spec.  The idea is to mangle:

    VLAT __attribute__((arm_sve_vector_bits(N)))

  as an instance __SVE_VLS<VLAT, N> of the template:

    __SVE_VLS<typename, unsigned>

- Give the fixed-length types their own TYPE_DECL.  This is needed
  to make the above mangling fix work, but should also be a minor
  QoI improvement for error reporting.  Unfortunately, the names are
  quite verbose, e.g.:

    svint8_t __attribute__((arm_sve_vector_bits(512)))

  but anything shorter would be ad-hoc syntax and so might be more
  confusing.

- Improve the error message reported when arm_sve_vector_bits is
  applied to tuples, such as:

    svint32x2_t __attribute__((arm_sve_vector_bits(N)))

  Previously we would complain that the type isn't an SVE type;
  now we complain that it isn't a vector type.

- Don't allow arm_sve_vector_bits(N) to be applied to existing
  fixed-length SVE types.

gcc/
	* config/aarch64/aarch64-sve-builtins.cc (add_sve_type_attribute):
	Take the ACLE name of the type as a parameter and add it as fourth
	argument to the "SVE type" attribute.
	(register_builtin_types): Update call accordingly.
	(register_tuple_type): Likewise.  Construct the name of the type
	earlier in order to do this.
	(get_arm_sve_vector_bits_attributes): New function.
	(handle_arm_sve_vector_bits_attribute): Report a more sensible
	error message if the attribute is applied to an SVE tuple type.
	Don't allow the attribute to be applied to an existing fixed-length
	SVE type.  Mangle the new type as __SVE_VLS<type, vector-bits>.
	Add a dummy TYPE_DECL to the new type.

gcc/testsuite/
	* g++.target/aarch64/sve/acle/general-c++/attributes_2.C: New test.
	* g++.target/aarch64/sve/acle/general-c++/mangle_6.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_7.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_8.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_9.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_10.C: Likewise.
	* gcc.target/aarch64/sve/acle/general/attributes_7.c: Check the
	error messages reported when arm_sve_vector_bits is applied to
	SVE tuple types or to existing fixed-length SVE types.