Fix FileCheck: substr() expect the length of the string as 2nd arg

The code assumed that substr() was taking start,end while it takes
start,length.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231988 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed