[AST][NFC] Order the bit-field classes of Stmt like in StmtNodes.td

Reorder the bit-field classes and the members of the anonymous union
so that they both match the order in StmtNodes.td.

There is already a fair amount of them, and this is not going to
improve. Therefore lets try to keep some order here.

Strictly NFC.

llvm-svn: 346793