This patch adds support for the P0482R5 core language changes.  This 
includes:
- The -fchar8_t and -fno_char8_t command line options.
- char8_t as a keyword.
- The char8_t builtin type as a non-aliasing unsigned integral
   character type of size 1.
- Use of char8_t as a simple type specifier.
- u8 character literals with type char8_t.
- u8 string literals with type array of const char8_t.
- User defined literal operators that accept char8_1 and char8_t pointer
   types.
- New __cpp_char8_t predefined feature test macro.
- New __CHAR8_TYPE__ and __GCC_ATOMIC_CHAR8_T_LOCK_FREE predefined
   macros .
- Name mangling and demangling for char8_t (using Du).

gcc/ChangeLog:

2018-11-04  Tom Honermann  <tom@honermann.net>

      * defaults.h: Define CHAR8_TYPE.

gcc/c-family/ChangeLog:

2018-11-04  Tom Honermann  <tom@honermann.net>
      * 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.
      * gcc/c-family/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-family/c-cppbuiltin.c (cpp_atomic_builtins): Predefine
      __GCC_ATOMIC_CHAR8_T_LOCK_FREE.
      (c_cpp_builtins): Predefine __cpp_char8_t.
      * c-family/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-family/c.opt: Add the -fchar8_t command line option.

gcc/c/ChangeLog:

2018-11-04  Tom Honermann  <tom@honermann.net>

      * c/c-typeck.c (char_type_p): Add char8_type_node.
      (digest_init): Handle initialization by a u8 string literal of
      char8_t type.

gcc/cp/ChangeLog:

2018-11-04  Tom Honermann  <tom@honermann.net>

      * cp/cvt.c (type_promotes_to): Handle char8_t promotion.
      * cp/decl.c (grokdeclarator): Handle invalid type specifier
      combinations involving char8_t.
      * cp/lex.c (init_reswords): Add char8_t as a reserved word.
      * cp/mangle.c (write_builtin_type): Add name mangling for char8_t
      (Du).
      * cp/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.
      * cp/rtti.c (emit_support_tinfos): type_info support for char8_t.
      * cp/tree.c (char_type_p): Recognize char8_t as a character type.
      * cp/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.
      * cp/typeck2.c (digest_init_r): Disallow initializing a char array
      with a u8 string literal.

libiberty/ChangeLog:

2018-10-31  Tom Honermann  <tom@honermann.net>
      * 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.

Tom.