[clang] Separate bit-field padding diagnostics into -Wpadded-bitfield (#70978)

The `-Wpadded` diagnostics are usually very noisy and generally not
helpful. However, reporting padding that was introduced in bit-fields is
rather helpful. For example, yesterday in SerenityOS's discord we had
very unpleasant experience of debugging Windows portability issue, and
its root cause was that under `x86_64-pc-windows-msvc` a padding was
introduced for one of the bit-fields.

So, this PR separates bit-field-related padding diagnostics into a new
`-Wpadded-bitfield`. The diagnostic group is also enabled by `-Wpadded`
for compatibility reasons.