[clang][dataflow] Tweak styling of iteration tabs. (#67637)

- On non-selected tabs, set cursor to 'pointer' to indicate they're
clickable

- Selected tab shares the same background as the contents to emphasize
that it's
associated with that content. (Compare with Google Chrome or VS Code,
which do
  this similarly.)

- Hovered-over tab becomes slightly brighter to indicate that clicking
it will
  make it even brighter (again, Chrome does this in a similar way).

Example:


![image](https://github.com/llvm/llvm-project/assets/29098113/e588068d-edff-49a8-b468-09dbb3255c3e)

(Mouse cursor is over "Iteration 2", but unfortunately I couldn't get
the screenshot to include the cursor.)