adds |operator=( const self_type& )| to nsSharable{C}String and nsXPIDL{C}String, to prevent the compiler from synthesizing its own (which won't do the right thing).
This should fix a bunch of XPIDLString-related cfront compile warnings.
original patch by jag, updated by me; r=dbaron, sr=jag.
git-svn-id: svn://10.0.0.236/trunk@144771 18797224-902f-48f8-a5cc-f745e15eee43