Add ID_cond expressions

[danpoe]

ID_case and ID_cond expressions are used in cbmc/develop but there are no corresponding expression types in std_expr.h. Moreover, ID_case and ID_cond are not assigned in cbmc/develop (they are assigned in EBMC).

New expression types should be added to std_expr.h and then the code in cbmc/develop should be refactored to use those instead of plain exprt.

[tautschnig]

Update: cond_exprt has been added in c4810ef. ID_case remains to be done.