Lex: Don't crash if both conflict markers are on the same line

We would check if the terminator marker is on a newline.  However, the
logic would end up out-of-bounds if the terminator marker immediately
follows the start marker.

This fixes PR21820.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224210 91177308-0d34-0410-b5e6-96231b3b80d8