Parse vars in classes the same as vars in other locations (#7188)

Class vars are still restricted to simple `name: type` bindings, not
full patterns. This is now handled in the check phase instead of during
parsing.

This is in preparation for supporting `static var`.

toolchain/check/pattern.cpp

@@ -6,10 +6,12 @@
 
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/check/action.h"
+#include "toolchain/check/class.h"
 #include "toolchain/check/control_flow.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/return.h"
 #include "toolchain/check/type.h"
+#include "toolchain/diagnostics/emitter.h"
 #include "toolchain/sem_ir/inst.h"
 
 namespace Carbon::Check {
@@ -134,6 +136,32 @@ auto AddBindingPattern(Context& context, SemIR::LocId name_loc,
   }
   auto type_id = SemIR::ExtractScrutineeType(context.sem_ir(), pattern.type_id);
 
+  // Handle `var` decls in a class by creating a `FieldDecl`.
+  if (context.full_pattern_stack().IsCurrentKindFieldDecl()) {
+    auto class_decl =
+        context.scope_stack().TryGetCurrentScopeAs<SemIR::ClassDecl>();
+    auto name_id = context.entity_names().Get(pattern.entity_name_id).name_id;
+    auto& class_info = context.classes().Get(class_decl->class_id);
+    auto field_type_id = GetUnboundElementType(
+        context, context.types().GetTypeInstId(class_info.self_type_id),
+        context.types().GetTypeInstId(type_id));
+
+    if (name_id == SemIR::NameId::Underscore) {
+      CARBON_DIAGNOSTIC(FieldNamedUnderscore, Error,
+                        "expected identifier in field declaration");
+      context.emitter().Emit(name_loc, FieldNamedUnderscore);
+    }
+
+    auto field_id =
+        AddInst<SemIR::FieldDecl>(context, name_loc,
+                                  {.type_id = field_type_id,
+                                   .name_id = name_id,
+                                   .index = SemIR::ElementIndex::None});
+    context.field_decls_stack().AppendToTop(field_id);
+
+    return {.pattern_id = field_id, .bind_id = field_id};
+  }
+
   auto bind_id = AddInstInNoBlock(
       context, SemIR::LocIdAndInst::RuntimeVerified(
                    context.sem_ir(), name_loc,