[clang][dataflow] Various formatting tweaks to HTMLLogger. (#68392)

*  Ensure timeline entries don't have linebreaks by setting
   `min-width: max-content`.

* Give the timeline header some padding on the right so it doesn't run
into the
   "Function" header.

* Put some padding around the iteration choosers so they don't run into
each
   other.

* Eliminate the "Iteration" text from the iteration choosers, which took
up a
lot of space. It should be clear what these are without the "Iteration"
text. (This makes the above problem of iteration choosers running into
each
   other less likely, but also add the padding nonetheless.)

Before:


![html_logger_before](https://github.com/llvm/llvm-project/assets/29098113/b938360f-c943-4552-829f-7b4857260959)

After:


![html_logger_after](https://github.com/llvm/llvm-project/assets/29098113/fecf4eab-7992-4e7d-a6e1-451887b699ab)