Change the datastructure for UnwrappedLines.

It was quite convoluted leading to us accidentally introducing O(N^2)
complexity while copying from UnwrappedLine to AnnotatedLine. We might
still want to improve the datastructure in AnnotatedLine (most
importantly not put them in a vector where they need to be copied on
vector resizing but that will be done as a follow-up.

This fixes most of the regression in llvm.org/PR14959.

No formatting changes intended.