[6.1] Fix an infinite loop if a conflict marker is found but it's not at the start of a new line

[ahoppen]

• Explanation: The parser would enter into an infinite loop when a closing conflict marker was not at the start of a line. Until I found the underlying issue, I was looking at performance issues with conflict marker lexing and was able to improve the parsing time of a file consisting of only conflict markers by 2x (not that it matters much in practice but doesn’t hurt either). That’s included in a second commit.
• Scope: Lexing of conflict markers
• Risk: Low
• Testing: Added a test case / manually tested performance of conflict marker parsing
• Issue: rdar://137746823
• Reviewer: @DougGregor @rintaro on Fix an infinite loop if a conflict marker is found but it's not at the start of a new line #2899

[ahoppen]

@swift-ci Please test