[CIR][NFC] Cleanup MissingFeature asserts in RecordLayoutBuilder (#161605)
This change cleans up some cir::MissingFeature asserts in
CIRGenRecordLayoutBuilder.cpp. In a couple of cases the asserts were
stale markers that we failed to remove when the correspond support was
implemented.
In one case, a cir::MissingFeature::bitfields() assert was ambiguous in
meaning and has been replaced by a comment and something more specific.
The missing feature in this case is just a bit of debug code to verify
certain bitfield-related conditions. This check should be added, but it
is not functionally required.