[clang2ir] Fix function name for retarded definitions

Partial revert of d2210718: the fix introduced in this commit
basically let template instances occur before the definition of the
actual parameter. With AST reordering in ir2clang, this should be less
of an issue as it once was, but for the time being I prefer to rely on
having some reordering also on the clang2ir side. As matter of fact, the
original issue was that we fail to set the name of [copyFunc] to a
fully qualified version, so that it didn't define the correct symbol:
we had two completely separate functions (i.e. test::func and ::func
in issue2564.cpp), one being actually unused.