Add a simulate_record_decl lang hook

Add a simulate_record_decl lang hook

This patch adds a lang hook for defining a struct/RECORD_TYPE
“as if” it had appeared directly in the source code.  It follows
the similar existing hook for enums.

It's the caller's responsibility to create the fields
(as FIELD_DECLs) but the hook's responsibility to create
and declare the associated RECORD_TYPE.

For now the hook is hard-coded to do the equivalent of:

  typedef struct NAME { FIELDS } NAME;

but this could be controlled by an extra parameter if some callers
want a different behaviour in future.

The motivating use case is to allow the long list of struct
definitions in arm_neon.h to be provided by the compiler,
which in turn unblocks various arm_neon.h optimisations.

gcc/
	* langhooks.h (lang_hooks_for_types::simulate_record_decl): New hook.
	* langhooks-def.h (lhd_simulate_record_decl): Declare.
	(LANG_HOOKS_SIMULATE_RECORD_DECL): Define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Include it.
	* langhooks.c (lhd_simulate_record_decl): New function.

gcc/c/
	* c-tree.h (c_simulate_record_decl): Declare.
	* c-objc-common.h (LANG_HOOKS_SIMULATE_RECORD_DECL): Override.
	* c-decl.c (c_simulate_record_decl): New function.

gcc/cp/
	* decl.c: Include langhooks-def.h.
	(cxx_simulate_record_decl): New function.
	* cp-objcp-common.h (cxx_simulate_record_decl): Declare.
	(LANG_HOOKS_SIMULATE_RECORD_DECL): Override.