Some versions of Clang apparently have an issue with
CastExpr::getSubExprAsWritten, causing false postivies as in
95787416 "loplugin:stringcopy", where
getSubExprAsWritten in

> CXXFunctionalCastExpr 0x114a333a8 'class rtl::OUString' functional cast to class rtl::OUString <ConstructorConversion>
> `-CXXBindTemporaryExpr 0x114a33388 'class rtl::OUString' (CXXTemporary 0x114a33380)
>   `-CXXConstructExpr 0x114a33348 'class rtl::OUString' 'void (class rtl::OUString &&)' elidable
>     `-MaterializeTemporaryExpr 0x114a33330 'class rtl::OUString' xvalue
>       `-CXXBindTemporaryExpr 0x114a33310 'class rtl::OUString' (CXXTemporary 0x114a33308)
>         `-ImplicitCastExpr 0x114a332f0 'class rtl::OUString' <UserDefinedConversion>
>           `-CXXMemberCallExpr 0x114a332c8 'class rtl::OUString'
>             `-MemberExpr 0x114a33290 '<bound member function type>' .operator OUString 0x1149b3b60
>               `-DeclRefExpr 0x114a324b8 'class jfw::CXmlCharPtr' lvalue Var 0x114a31ce0 'sUser' 'class jfw::CXmlCharPtr'

erroneously returns the MaterializeTemporaryExpr instead of looking through all
the way down to the DeclRefExpr.  Will need further investigation.

Change-Id: I579cd6047b8bbf8833123ce5ad47ae7e3a33eb12