debuginfo: Add temporary workaround for #14871.
auto merge of #14909 : michaelwoerister/rust/typemap-workaround, r=pnkfelix

Whenever something is added to the debuginfo::TypeMap there's a consistency check, making sure that the code does what it is supposed to do. For some types (e.g. slices in a recursion cycle) this check rightfully fails. However, there's no need to actually abort compilation as the inconsistency won't lead to corrupt state, due to LLVM metadata uniquing.

A later, more thorough fix (coming next week) should remove the inconsistency altogether and also make this check non-fatal. For the time being, omit the check so people can compile their code with -g.