Add a generic lhd_simulate_enum_decl

Add a generic lhd_simulate_enum_decl

Normally we only create SVE ACLE functions when arm_sve.h is included.
But for LTO we need to do it at start-up, so that the functions are
already defined when streaming in the LTO objects.

One hitch with doing that is that LTO doesn't yet implement the
simulate_enum_decl langhook.  This patch adds a simple default
implementation that it can use.

2020-01-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* langhooks-def.h (lhd_simulate_enum_decl): Declare.
	(LANG_HOOKS_SIMULATE_ENUM_DECL): Use it.
	* langhooks.c: Include stor-layout.h.
	(lhd_simulate_enum_decl): New function.
	* config/aarch64/aarch64-sve-builtins.cc (init_builtins): Call
	handle_arm_sve_h for the LTO frontend.
	(register_vector_type): Cope with null returns from pushdecl.

gcc/testsuite/
	* gcc.target/aarch64/sve/pcs/asm_4.c: New test.

From-SVN: r279954