Add a simulate_enum_decl langhook

Add a simulate_enum_decl langhook

Similarly to the simulate_builtin_function_decl patch, this one
adds a hook for simulating an enum declaration in the source
language.  Again, the main SVE ACLE patch has tests for various
error conditions.

2019-10-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* coretypes.h (string_int_pair): New typedef.
	* langhooks-def.h (LANG_HOOKS_SIMULATE_ENUM_DECL): Define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Include it.
	* langhooks.h (lang_hooks_for_types::simulate_enum_decl): New hook.

gcc/c/
	* c-tree.h (c_simulate_enum_decl): Declare.
	* c-decl.c (c_simulate_enum_decl): New function.
	* c-objc-common.h (LANG_HOOKS_SIMULATE_ENUM_DECL): Define to the above.

gcc/cp/
	* cp-objcp-common.h (cxx_simulate_enum_decl): Declare.
	(LANG_HOOKS_SIMULATE_ENUM_DECL): Define to the above.
	* decl.c (cxx_simulate_enum_decl): New function.

From-SVN: r277555