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>
llvm-svn: 231988
diff --git a/llvm/utils/FileCheck/FileCheck.cpp b/llvm/utils/FileCheck/FileCheck.cpp
index 146b512..f7c0524 100644
--- a/llvm/utils/FileCheck/FileCheck.cpp
+++ b/llvm/utils/FileCheck/FileCheck.cpp
@@ -1053,7 +1053,6 @@
     PrintCheckFailed(SM, *this, MatchBuffer, VariableTable);
     return StringRef::npos;
   }
-  MatchPos += LastPos;
 
   // Similar to the above, in "label-scan mode" we can't yet handle CHECK-NEXT
   // or CHECK-NOT
@@ -1076,7 +1075,7 @@
       return StringRef::npos;
   }
 
-  return MatchPos;
+  return LastPos + MatchPos;
 }
 
 bool CheckString::CheckNext(const SourceMgr &SM, StringRef Buffer) const {