[libc] Add a test condition for EOF in ctype

[xgupta]

Summary

This address https://github.com/llvm/llvm-project/issues/62000.

Header stdio.h is included because EOF define there.
The loop condition changed from <255 to <=255 to include EOF.

[xgupta]

Update < x256 to <= 255

[xgupta]

Abandoned in favor of https://reviews.llvm.org/D147813.

libc/test/src/ctype/isalpha_test.cpp

 //===-- Unittests for isalpha----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
+#include <stdio.h>
+
 #include "src/ctype/isalpha.h"
 
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcIsAlpha, DefaultLocale) {
   // Loops through all characters, verifying that letters return a
-  // non-zero integer and everything else returns zero.
-  for (int ch = 0; ch < 255; ++ch) {
-    if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z'))
+  // non-zero integer and everything else including EOF returns zero.
+  for (int ch = 0; ch <= 255; ++ch) {
+    if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z')) {
       EXPECT_NE(__llvm_libc::isalpha(ch), 0);
-    else
+    } else if (ch == EOF) {
+      EXPECT_EQ(__llvm_libc::isalpha(ch), 0);
+    } else {
       EXPECT_EQ(__llvm_libc::isalpha(ch), 0);
     }
   }
+}