infcall, c++: allow more info to be computed for pass-by-reference values
In C++, call-by-value arguments that cannot be trivially copied are
implicitly passed by reference. When making an infcall, GDB needs to
find out if an argument is pass-by-reference or not, so that the
correct semantics can be followed. This patch enriches the
information computed by the language ops for pass-by-reference
arguments. Instead of a plain binary result, the computed information
now includes whether the argument is