* cp-error.def: New file.
* Make-lang.in (cc1plus): Add dependency on it.
* Makefile.in (CXX_TREE_H): Likewise.
* cp-tree.h (error_code): New type.
Include cp-error.def to get error codes. Make it possible to
include cp-tree.h in errfn.c.
(enable_warning): New function.
(flag_diag_codes): New variable.
* errfn.c: Include cp-tree.h.
(cp_err_msg): New type. Include cp-error.def to define it.
(err_msgs): New variable.
(is_warning_enabled): New function.
(cp_thing): Take an error_code, not a format string. Output the
diagnostic code if flag_diag_codes.
(cp_error, cp_compiler_error, cp_sprintf, cp_error_at): Take
error_codes, not format strings.
(cp_warning, cp_pedwarn, cp_warning_at, cp_pedwarn_at): Likewise.
Call is_warning_enabled.
* decl2.c (flag_diag_codes): New variable.
(lang_decode_option): Handle -fdiag-codes and -Wnumber.
* lang-options.h: Add -fdiag-codes, -fno-diag-codes, and
-W-number-, and -Wno-number.
* class.c: Update all calls to error-functions.
* cvt.c: Likewise.
* decl.c: Likewise.
* decl2.c: Likewise.
* except.c: Likewise.
* expr.c: Likewise.
* friend.c: Likewise.
* lex.c: Likewise.
* parse.y: Likewise.
* pt.c: Likewise.
* repo.c: Likewise.
* rtti.c: Likewise.
* search.c: Likewise.
* semantics.c: Likewise.
* sig.c: Likewise.
* tree.c: Likewise.
* typeck.c: Likewise.
* typeck2.c: Likewise.
* xref.c: Likewise.