[cxx-interop] Fix codegen for base member accesses
The code path inserted a manual lvalue to rvalue conversion. This
conversion triggered a bitwise copy operation that should not be used
for non-trivial types (a CXXConstructExpr is required instead). Instead
of manually trying to do the casts, let's rely on Clang's sema to figure
out the right casts to instert. Also, the original code used an rvalue
reference as the return type for the synthesized getter. This triggered
some compilation errors in sema's buildReturnStmt. Returning an lvalue
reference solved those errors and is more in line with wath we actually
want to do here (since we do not really need the getter/setter to
produce an rvalue reference, we do not want to move out of the field).
rdar://136757628
