make field count assertion on `pretty_print` configurable

[mofeing]

i've a pipeline that correctly translates some C++ classes but unfortunately flattens the fields inherited from the parent classes (note: this might do to a translation from C++ to C prior to Clang.jl).

i have some code that detects those fields and replaces the fields for just one field of the parent type (i.e. forces inheritance through composition), but Clang.jl throws ERROR: AssertionError: length(child_nodes) == length(fields).

this PR makes that assertion optional.