[ClangImporter] Fix IUO ordering bug

lib/Sema/TypeCheckDecl.cpp

@@ -1854,6 +1854,20 @@ IsImplicitlyUnwrappedOptionalRequest::evaluate(Evaluator &evaluator,
   }
 
   case DeclKind::Var:
+    if (decl->hasClangNode()) {
+      // ClangImporter does not use this request to compute whether imported
+      // declarations are IUOs; instead, it explicitly sets the bit itself when
+      // it imports the declaration's type. For most declarations this is done
+      // greedily, but for VarDecls, it is deferred until `getInterfaceType()`
+      // is called for the first time. (See apple/swift#61026.)
+      //
+      // Force the interface type, then see if a result for this request is now
+      // cached.
+      // FIXME: This is a little gross.
+      (void)decl->getInterfaceType();
+      if (auto cachedResult = this->getCachedResult())
+        return *cachedResult;
+    }
     TyR = cast<VarDecl>(decl)->getTypeReprOrParentPatternTypeRepr();
     break;
 

test/api-digester/Inputs/ImportedIUO/imported_iuo.h

@@ -0,0 +1 @@
+void *ImportedIUOVar;

test/api-digester/Inputs/ImportedIUO/module.modulemap

@@ -0,0 +1,3 @@
+module ImportedIUO {
+  header "imported_iuo.h"
+}

test/api-digester/imported_iuo.test

@@ -0,0 +1,11 @@
+RUN: %empty-directory(%t)
+RUN: %api-digester -dump-sdk -module ImportedIUO -o %t/digest.json -module-cache-path %t/module-cache %clang-importer-sdk-nosource -I %S/Inputs/ImportedIUO -avoid-location
+RUN: %FileCheck --input-file %t/digest.json %s
+
+CHECK:        "name": "ImportedIUOVar",
+CHECK-NEXT:        "printedName": "ImportedIUOVar",
+CHECK-NEXT:        "children": [
+CHECK-NEXT:          {
+CHECK-NEXT:            "kind": "TypeNominal",
+CHECK-NEXT:            "name": "ImplicitlyUnwrappedOptional",
+CHECK-NEXT:            "printedName": "Swift.UnsafeMutableRawPointer!",