c-index-test.c: Fix cygwin warning not to pass signed char to islower(c).

Cygwin's ctype.h says;
/* These macros are intentionally written in a manner that will trigger
   a gcc -Wall warning if the user mistakenly passes a 'char' instead
   of an int containing an 'unsigned char'.
   (snip) */

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127308 91177308-0d34-0410-b5e6-96231b3b80d8

tools/c-index-test/c-index-test.c

@@ -990,7 +990,7 @@ void print_completion_result(CXCompletionResult *completion_result,
 
 int my_stricmp(const char *s1, const char *s2) {
   while (*s1 && *s2) {
-    int c1 = tolower(*s1), c2 = tolower(*s2);
+    int c1 = tolower((unsigned char)*s1), c2 = tolower((unsigned char)*s2);
     if (c1 < c2)
       return -1;
     else if (c1 > c2)