[clang][Diagnostics][NFC] Don't create oversized CaretLine

Instead of creating a CaretLine the size of the SourceLine, just leave
it empty at first, let HighlightRange resize it to fit all the ~, then
resize it to fit the ^. Then we can save ourselves the work to remove
the trailing whitespace again.

Differential Revision: https://reviews.llvm.org/D151286