[AST] Don't consider 'ExportDecl' when calculating DeclContext 'Encloses'

This mimics the style of 90010c2e1 (Don't consider 'LinkageSpec' when
calculating DeclContext 'Encloses'). Since ExportDecl and LinkageSpec
are transparent DeclContext, they share some similarity.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D116911

clang/lib/AST/DeclBase.cpp

@@ -1212,7 +1212,8 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
     return getPrimaryContext()->Encloses(DC);
 
   for (; DC; DC = DC->getParent())
-    if (!isa<LinkageSpecDecl>(DC) && DC->getPrimaryContext() == this)
+    if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+        DC->getPrimaryContext() == this)
       return true;
   return false;
 }

clang/test/SemaCXX/lookup-through-export.cpp

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -std=c++20 %s -verify
+
+// expected-no-diagnostics
+export module X;
+export {
+  namespace A {
+  namespace B {
+  int bar;
+  }
+  } // namespace A
+  namespace C {
+  void foo() {
+    using namespace A;
+    (void)B::bar;
+  }
+  } // namespace C
+}
+
+export {
+  namespace D {
+  namespace E {
+  int bar;
+  }
+  } // namespace D
+  namespace F {
+  void foo() {
+    using namespace D;
+    (void)E::bar;
+  }
+  } // namespace F
+}