Add flags check to createVariantMemberType (#139261)
I noticed that DIDerivedType overloads the "ExtraData" member depending
on the precise type being implemented. A variant part uses this to store
the discriminant (a reference to another member), but a bit field uses
it to store the storage offset.
This patch changes createVariantMemberType to ensure that the
FlagBitField is not used when creating a variant part. If this flag is
used, the ExtraData field would be erroneously used in two different
ways.
The patch also updates a comment in DIDerivedType to list a couple more
cases.
DIBuilder.cpp