codegen: split gc roots from other bits on stack (#55767)
In order to help avoid memory provenance issues, and better utilize
stack space (somewhat), and use FCA less, change the preferred
representation of an immutable object to be a pair of
`<packed-data,roots>` values. This packing requires some care at the
boundaries and if the expected field alignment exceeds that of a
pointer. The change is expected to eventually make codegen more flexible
at representing unions of values with both bits and pointer regions.
Eventually we can also have someone improve the late-gc-lowering pass to
take advantage of this increased information accuracy, but currently it
will not be any better than before at laying out the frame.
