rust: Fix ICE in function_set_parameters

[riogu]

When lowering a function to HIR with a refutable pattern in its parameter pattern, we were ICE'ing due to a lack of checks for invalid refutable patterns. Fix this by erroring on invalid patterns while lowering a function.

Fixes #3919

gcc/rust/ChangeLog:

* hir/rust-ast-lower-item.cc (ASTLoweringItem::visit): added refutable pattern checks for function params.

gcc/testsuite/ChangeLog:

* rust/compile/issue-3919-ice-func-parms.rs: New test.

[CohenArthur]

Thank you, that's good work! I'm not sure that we'll keep that check here in the lowerer but it definitely improves the compiler for now, so I'm in favor of merging it :)

[riogu]

Thank you, that's good work! I'm not sure that we'll keep that check here in the lowerer but it definitely improves the compiler for now, so I'm in favor of merging it :)

I wasn't sure where to place it more naturally before the lowering phase, but i felt it would be an improvement anyway.
It might be interesting to add support for Irrefutability checks, since i noticed that there is an overall lack for these, and could be implemented as mentioned in this comment in rust-ast.h and rust-hir-pattern-abstract.h, and also mentioned by #2082.

// Pattern base HIR node
class Pattern : public Node, virtual public FullVisitable
{
  ...
  // possible virtual methods: is_refutable()
  ...
}

doing that would replace this fix with an equivalent solution calling is_refutable(), but would require implementing all the derived cases.