enum declaration for StrKind, ExprKind

Reduce header include, but violate `pedantic`.

CIで失敗：

```
cc -ansi -std=c11 -pedantic -MMD -Wall -Wextra -Werror -Wold-style-definition -Wno-missing-field-initializers -Wno-empty-body -D_DEFAULT_SOURCE -O2 -g3 -Isrc/util -DXCC_TARGET_ARCH=XCC_ARCH_X64 -Isrc/cc/frontend -c -o obj/initializer.o src/cc/frontend/initializer.c
In file included from src/cc/frontend/expr.c:2:
src/cc/frontend/expr.h:22:68: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   22 | Expr *string_expr(const Token *token, char *str, ssize_t len, enum StrKind kind);
      |                                                                    ^~~~~~~
src/cc/frontend/expr.h:32:29: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   32 | Expr *new_expr_num_bop(enum ExprKind kind, const Token *tok, Expr *lhs, Expr *rhs);
      |                             ^~~~~~~~
src/cc/frontend/expr.h:33:29: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   33 | Expr *new_expr_int_bop(enum ExprKind kind, const Token *tok, Expr *lhs, Expr *rhs);
      |                             ^~~~~~~~
src/cc/frontend/expr.h:34:28: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   34 | Expr *new_expr_addsub(enum ExprKind kind, const Token *tok, Expr *lhs, Expr *rhs);
      |                            ^~~~~~~~
src/cc/frontend/expr.h:41:22: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   41 | Expr *incdec_of(enum ExprKind kind, Expr *target, const Token *tok);
      |                      ^~~~~~~~
src/cc/frontend/expr.h:42:25: error: ISO C forbids forward references to ‘enum’ types [-Werror=pedantic]
   42 | Expr *new_expr_cmp(enum ExprKind kind, const Token *tok, Expr *lhs, Expr *rhs);
      |                         ^~~~~~~~
```

Author: tyfkda

src/cc/frontend/expr.h

@@ -1,6 +1,23 @@
 #pragma once
 
-#include "ast.h"
+#include <stdbool.h>
+#include <stddef.h>  // size_t
+#include <sys/types.h>  // ssize_t
+
+typedef struct Expr Expr;
+typedef struct MemberInfo MemberInfo;
+typedef struct Token Token;
+typedef struct Type Type;
+
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+#endif
+enum ExprKind;
+enum StrKind;
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
 Expr *string_expr(const Token *token, char *str, ssize_t len, enum StrKind kind);
 Expr *calc_type_size(const Type *type);

src/cc/frontend/type.c

@@ -242,7 +242,7 @@ bool is_unsigned(const Type *type) {
   return (is_fixnum(type) && type->fixnum.is_unsigned) || type->kind == TY_PTR;
 }
 
-bool is_char_type(const Type *type, /*enum FixnumKind*/int str_kind) {
+bool is_char_type(const Type *type, enum StrKind str_kind) {
   switch ((enum StrKind)str_kind) {
   case STR_CHAR:
     return type->kind == TY_FIXNUM && type->fixnum.kind == FX_CHAR;

src/cc/frontend/type.h

@@ -12,6 +12,15 @@ typedef struct Expr Expr;
 typedef struct Name Name;
 typedef struct Vector Vector;
 
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+#endif
+enum StrKind;
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
 // Fixnum
 
 enum FixnumKind {
@@ -156,7 +165,7 @@ static inline bool is_flonum(const Type *type)  {
 static inline bool is_bool(const Type *type)  { return type->kind == TY_FIXNUM && type->fixnum.kind == FX_BOOL; }
 bool is_number(const Type *type);
 bool is_unsigned(const Type *type);
-bool is_char_type(const Type *type, /*enum StrKind*/int str_kind);
+bool is_char_type(const Type *type, enum StrKind str_kind);
 bool is_void_ptr(const Type *type);
 bool is_prim_type(const Type *type);
 static inline bool ptr_or_array(const Type *type)  { return type->kind == TY_PTR || type->kind == TY_ARRAY; }