lower: Error out when lowering ErrorPropagationExpr

Adapt functions for lowering nodes that should never reach the lowering phase to cause an
unreachable, and mark them as final so as it not possible to override them in other visitors.

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-base.cc: Adapt functions for ErrorPropagationExpr and MacroInvocation.
	* hir/rust-ast-lower-base.h: Mark them as final.
	* hir/rust-ast-lower-expr.cc: Remove previous definition for those overrides.
	* hir/rust-ast-lower-expr.h: Likewise.

Author: CohenArthur

gcc/rust/hir/rust-ast-lower-base.cc

@@ -23,21 +23,29 @@
 #include "rust-ast.h"
 #include "rust-attribute-values.h"
 #include "rust-diagnostics.h"
+#include "rust-expr.h"
 #include "rust-item.h"
 #include "rust-system.h"
 #include "rust-attributes.h"
 
 namespace Rust {
 namespace HIR {
 
-// We special case lowering macro invocations as that should NEVER happen
 void
 ASTLoweringBase::visit (AST::MacroInvocation &invoc)
 {
   rust_fatal_error (invoc.get_locus (), "rogue macro detected during lowering");
   rust_unreachable ();
 }
 
+void
+ASTLoweringBase::visit (AST::ErrorPropagationExpr &expr)
+{
+  rust_fatal_error (expr.get_locus (),
+		    "missing desugar for question mark operator");
+  rust_unreachable ();
+}
+
 void
 ASTLoweringBase::visit (AST::Token &)
 {}
@@ -116,9 +124,6 @@ void
 ASTLoweringBase::visit (AST::DereferenceExpr &)
 {}
 void
-ASTLoweringBase::visit (AST::ErrorPropagationExpr &)
-{}
-void
 ASTLoweringBase::visit (AST::NegationExpr &)
 {}
 void