[clang] [clang:frontend] Add `-Wbit-cast-padding` default warning (PR #200362)

[Jan Schultke via cfe-commits]

eisenwave wrote:

> We already have code for finding padding (see #75371); can we share the code?

Oh wow, yeah, it looks like we can. The only issue is that the `PaddingClearer` only gives you padding intervals but not union intervals (not sure why), but finding the union intervals is relatively trivial compared to finding padding intervals.

https://github.com/llvm/llvm-project/pull/200362