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) */

llvm-svn: 127308