Two problems with gimplify_init_constructor

[Richard Kenner]

    > Should we just do the same thing as for RTL with the corresponding check?
    
    You mean clear TREE_READONLY?  Yes.

Actually, I meant suppress the clearing operation in the readonly case.
There are three cases and in only one can we clear TREE_READONLY: when it's
set in a decl.  In the other cases (readonly type and a type having readonly
fields), we have to suppress the separate clear.  Indeed I'm not even sure
it's worth clearing TREE_READONLY in the case where only it's set.