ensure sub-bitfield bits are always defined [NFC]

[vtjnash]

Helps avoid UB or inconsistencies between Julia and C definitions.

[gbaraldi]

Why are we adding the padding in the taggedvalue_bits if we don't set them?(I know setting them would be incorrect) but I thought that union would be fine here.

[vtjnash]

Adding padding forces C to consider them preserved after changing the other bits. It doesn't seem like the compiler usually bothers to corrupt them, but the bits were technically undefined after changing them because they are in the union.

[gbaraldi]

Ok, then LGTM