[AArch64] Fix build for non-default languages

[AArch64] Fix build for non-default languages

The SVE PCS support broke go, D and Ada because those languages don't
call TARGET_INIT_BUILTINS.  We therefore ended up trying to get the
TYPE_MAIN_VARIANT of a null __SVBool_t.

We shouldn't really need to apply TYPE_MAIN_VARIANT there anyway,
since the ABI-defined types are (and need to be) their own main
variants.  This patch asserts for that instead.

2019-10-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve-builtins.cc (register_builtin_types):
	Assert that the type we store in abi_vector_types is its own
	main variant.
	(svbool_type_p): Don't apply TYPE_MAIN_VARIANT here.

From-SVN: r277680