[llvm-cov] Simplify the way expansion views are rendered (NFC)

If a sub-view has already been rendered, it's helpful to re-render the
expansion site before rendering the next expansion view. Make this fact
explicit in the rendering interface, instead of hiding it behind an
awkward Optional<LineRef> parameter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273789 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed