[MC] Reduce size of MCDataFragment by 8 bytes (#95293)

Due to alignment, the first two fields of MCEncodedFragment are
currently at bytes 40 and 41, so 1 byte over the 8 byte boundary,
causing 7 bytes padding to be inserted for the following pointer.

Fold two bools of MCFragment into bitfields to reduce move the two
fields of MCEncodedFragment one byte earlier to remove the padding
bytes. This works, as in the Itanium ABI, there is no padding after
base classes.

This gives a space reduction of MCDataFragment from 224 to 216 bytes.