gccrs: Fix ICE when handling case of unknown field in HIR::FieldAccess

gccrs: Fix ICE when handling case of unknown field in HIR::FieldAccess

We were wrongly adding the assertion that this must not be an enum but
this is a pointless assertion we only care that there are variant in the
ADT and if the field exists in the first variant.

Fixes Rust-GCC#3581

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): fix bad assertion

gcc/testsuite/ChangeLog:

	* rust/compile/nonexistent-field.rs: fix bad error message
	* rust/compile/issue-3581-1.rs: New test.
	* rust/compile/issue-3581-2.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>