aarch64: Fix SVE ACLE builtins with LTO [PR99216]

As discussed in the PR, we currently have two different numbering
schemes for SVE builtins: one for C, and one for C++. This is
problematic for LTO, where we end up getting confused about which
intrinsic we're talking about. This patch inserts placeholders into the
registered_functions vector to ensure that there is a consistent
numbering scheme for both C and C++.

We use integer_zero_node as a placeholder node instead of building a
function decl. This is safe because the node is only returned by the
TARGET_BUILTIN_DECL hook, which (on AArch64) is only used for validation
when builtin decls are streamed into lto1.

gcc/ChangeLog:

	PR target/99216
	* config/aarch64/aarch64-sve-builtins.cc
	(function_builder::add_function): Add placeholder_p argument, use
	placeholder decls if this is set.
	(function_builder::add_unique_function): Instead of conditionally adding
	direct overloads, unconditionally add either a direct overload or a
	placeholder.
	(function_builder::add_overloaded_function): Set placeholder_p if we're
	using C++ overloads. Use the obstack for string storage instead
	of relying on the tree nodes.
	(function_builder::add_overloaded_functions): Don't return early for
	m_direct_overloads: we need to add placeholders.
	* config/aarch64/aarch64-sve-builtins.h
	(function_builder::add_function): Add placeholder_p argument.

gcc/testsuite/ChangeLog:

	PR target/99216
	* g++.target/aarch64/sve/pr99216.C: New test.

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