Define `HAVE___BUILTIN_UNREACHABLE` instead of `UNREACHABLE`

`UNREACHABLE` in ruby/internal/has/builtin.h is only used as just
a flag now, and redefined in ruby/backward/2/assume.h then.

configure.ac

@@ -1677,7 +1677,7 @@ AS_IF([test "$GCC" = yes], [
     ])
     ])
     AS_IF([test "$rb_cv_func___builtin_unreachable" = yes], [
-	AC_DEFINE_UNQUOTED(UNREACHABLE, [__builtin_unreachable()])
+	AC_DEFINE(HAVE___BUILTIN_UNREACHABLE)
     ])
 }
 

include/ruby/backward/2/assume.h

@@ -25,7 +25,6 @@
 #include "ruby/internal/has/builtin.h"
 
 #undef  ASSUME             /* Kill config.h definition */
-#undef  UNREACHABLE        /* Kill config.h definition */
 #define ASSUME             RBIMPL_ASSUME             /**< @old{RBIMPL_ASSUME} */
 #define UNREACHABLE        RBIMPL_UNREACHABLE()      /**< @old{RBIMPL_UNREACHABLE} */
 #define UNREACHABLE_RETURN RBIMPL_UNREACHABLE_RETURN /**< @old{RBIMPL_UNREACHABLE_RETURN} */

include/ruby/internal/has/builtin.h

@@ -78,10 +78,6 @@
 # define RBIMPL_HAS_BUILTIN___builtin_unreachable       RBIMPL_COMPILER_SINCE(GCC, 4, 5, 0)
 # /* Note that "0, 0, 0" might be inaccurate. */
 
-#elif RBIMPL_COMPILER_IS(MSVC)
-# /* MSVC has UNREACHABLE, but that is not __builtin_unreachable. */
-# define RBIMPL_HAS_BUILTIN(_) 0
-
 #else
 # /* Take config.h definition when available */
 # define RBIMPL_HAS_BUILTIN(_) ((RBIMPL_HAS_BUILTIN_ ## _)+0)
@@ -111,7 +107,7 @@
 # define RBIMPL_HAS_BUILTIN___builtin_rotateright64     0
 # define RBIMPL_HAS_BUILTIN___builtin_popcountll        HAVE_BUILTIN___BUILTIN_POPCOUNTLL
 # define RBIMPL_HAS_BUILTIN___builtin_sub_overflow      HAVE_BUILTIN___BUILTIN_SUB_OVERFLOW
-# if defined(UNREACHABLE)
+# if defined(HAVE___BUILTIN_UNREACHABLE)
 #  define RBIMPL_HAS_BUILTIN___builtin_unreachable 1
 # else
 #  define RBIMPL_HAS_BUILTIN___builtin_unreachable 0

win32/Makefile.sub

@@ -702,7 +702,6 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define ALWAYS_INLINE(x) __forceinline x
 #define WARN_UNUSED_RESULT(x) x
 #define MAYBE_UNUSED(x) x
-#define UNREACHABLE __assume(0)
 #define ASSUME(x) __assume(!!(x))
 #define FUNC_STDCALL(x) __stdcall x
 #define FUNC_CDECL(x) __cdecl x