C: Fix ListConcat and ListSection

[Shaikh-Ubaid]

fixes #2223

[Shaikh-Ubaid]

With respect to C_CPP: Return value instead of pointer in list_concat, it seems returning value is a better approach. It simplifies the logic for handling return value of list_concat, as we can see in the commit.

A value returned is also helpful in visit_ListLen():

lpython/src/libasr/codegen/asr_to_c_cpp.h

Line 1691 in 638b052

src = src + ".current_end_point";

visit_ListSection() already returns a value:

lpython/src/libasr/codegen/asr_to_c_cpp.h

Line 1623 in 638b052

src = "* " + var_name;

But it is not appropriately casted (using brackets) which is fixed in commit C_CPP: Fix list section in this PR.

[certik]

I think this is fine.