Implement P0482R5, char8_t: A type for UTF-8 characters and strings

Implement P0482R5, char8_t: A type for UTF-8 characters and strings

gcc/cp/
	* cvt.c (type_promotes_to): Handle char8_t promotion.
	* decl.c (grokdeclarator): Handle invalid type specifier
	combinations involving char8_t.
	* lex.c (init_reswords): Add char8_t as a reserved word.
	* mangle.c (write_builtin_type): Add name mangling for char8_t (Du).
	* parser.c (cp_keyword_starts_decl_specifier_p)
	(cp_parser_simple_type_specifier): Recognize char8_t as a simple
	type specifier.
	(cp_parser_string_literal): Use char8_array_type_node for the type
	of CPP_UTF8STRING.
	(cp_parser_set_decl_spec_type): Tolerate char8_t typedefs in system
	headers.
	* rtti.c (emit_support_tinfos): type_info support for char8_t.
	* tree.c (char_type_p): Recognize char8_t as a character type.
	* typeck.c (string_conv_p): Handle conversions of u8 string
	literals of char8_t type.
	(check_literal_operator_args): Handle UDLs with u8 string literals
	of char8_t type.
	* typeck2.c (ordinary_char_type_p): New.
	(digest_init_r): Disallow initializing a char array with a u8 string
	literal.
gcc/c-family/
	* c-common.c (c_common_reswords): Add char8_t.
	(fix_string_type): Use char8_t for the type of u8 string literals.
	(c_common_get_alias_set): char8_t doesn't alias.
	(c_common_nodes_and_builtins): Define char8_t as a builtin type in
	C++.
	(c_stddef_cpp_builtins): Add __CHAR8_TYPE__.
	(keyword_begins_type_specifier): Add RID_CHAR8.
	* c-common.h (rid): Add RID_CHAR8.
	(c_tree_index): Add CTI_CHAR8_TYPE and CTI_CHAR8_ARRAY_TYPE.
	Define D_CXX_CHAR8_T and D_CXX_CHAR8_T_FLAGS.
	Define char8_type_node and char8_array_type_node.
	* c-cppbuiltin.c (cpp_atomic_builtins): Predefine
	__GCC_ATOMIC_CHAR8_T_LOCK_FREE.
	(c_cpp_builtins): Predefine __cpp_char8_t.
	* c-lex.c (lex_string): Use char8_array_type_node as the type of
	CPP_UTF8STRING.
	(lex_charconst): Use char8_type_node as the type of CPP_UTF8CHAR.
	* c-opts.c: If not otherwise specified, enable -fchar8_t when
	targeting C++2a.
	* c.opt: Add the -fchar8_t command line option.
libiberty/
	* cp-demangle.c (cplus_demangle_builtin_types)
	(cplus_demangle_type): Add name demangling for char8_t (Du).
	* cp-demangle.h: Increase D_BUILTIN_TYPE_COUNT to accommodate the
	new char8_t type.

From-SVN: r267923