Lkt lowering: add AnalysisUnit type handling

This is needed for the test in the next commit to work.

TN: RA22-015

Author: pmderodat

contrib/lkt/language/parser.py

@@ -227,6 +227,18 @@ def get_builtin_gen_decl(entity_name=T.Symbol):
         doc="Unit method. Return the Iterator builtin trait."
     )
 
+    analysis_unit_gen_trait = Property(
+        Self.get_builtin_gen_decl('AnalysisUnit'),
+        public=True,
+        doc="Unit method. Return the ``AnalysisUnit`` builtin generic trait."
+    )
+
+    analysis_unit_trait = Property(
+        Self.analysis_unit_gen_trait.decl.cast(T.TraitDecl),
+        public=True,
+        doc="Unit method. Return the ``AnalysisUnit`` builtin trait."
+    )
+
     @langkit_property(external=True,
                       uses_entity_info=False,
                       uses_envs=True,

langkit/lkt_lowering.py

@@ -1161,6 +1161,7 @@ def get_field(decl: L.TraitDecl, name: str) -> L.Decl:
         self.astlist_type = root.p_astlist_type
         self.iterator_trait = root.p_iterator_trait
         self.map_method = get_field(self.iterator_trait, 'map')
+        self.analysis_unit_trait = root.p_analysis_unit_trait
 
         # Map Lkt nodes for the declarations of builtin types to the
         # corresponding CompiledType instances.
@@ -1234,6 +1235,9 @@ def resolve_type_decl(self, decl: L.TypeDecl) -> CompiledTypeOrDefer:
                 assert isinstance(node, (ASTNodeType, TypeRepo.Defer))
                 result = node.list
 
+            elif inner_type == self.analysis_unit_trait:
+                result = T.AnalysisUnit
+
             else:
                 assert False, (
                     'Unknown generic type: {} (from {})'