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
1 file changed