[cxx-interop] Fix unqualified name lookup failure

When MemberImportVisibility is enabled we failed to find certain base
methods from the extensions when said base methods are imported from
C++.

rdar://154887575

Author: Gabor Horvath

lib/AST/NameLookup.cpp

@@ -2423,7 +2423,15 @@ static bool isAcceptableLookupResult(const DeclContext *dc, NLOptions options,
     // Check that there is some import in the originating context that makes
     // this decl visible.
     if (!(options & NL_IgnoreMissingImports)) {
-      if (!dc->isDeclImported(decl))
+      bool isFromClangModule = false;
+      auto definingModule = decl->getModuleContextForNameLookup();
+      if (dc->getASTContext().LangOpts.EnableCXXInterop) {
+        if (definingModule->isClangHeaderImportModule())
+          isFromClangModule = true;
+      }
+      // dc->isDeclImported(decl) false even when the correct clang module is
+      // imported.
+      if (!dc->isDeclImported(decl) && !isFromClangModule)
         return false;
     }
 

test/Interop/Cxx/class/inheritance/Inputs/inherited-lookup.h

@@ -16,3 +16,12 @@ struct IIOne : IOne {
 struct IIIOne : IIOne {
   int methodIII(void) const { return -111; }
 };
+
+class Base {
+public:
+  bool baseMethod() const { return true; }
+};
+
+namespace Bar {
+class Derived : public Base {};
+} // namespace Bar

test/Interop/Cxx/class/inheritance/inherited-lookup-executable.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -cxx-interoperability-mode=default)
+// RUN: %target-run-simple-swift(-I %S/Inputs/ -Xfrontend -cxx-interoperability-mode=default -enable-upcoming-feature MemberImportVisibility)
 //
 // REQUIRES: executable_test
 import InheritedLookup
@@ -62,4 +62,15 @@ InheritedMemberTestSuite.test("Eagerly imported methods resolve to base classes"
   expectEqual(iiiOne[0], 1)
 }
 
+extension Bar.Derived {
+    public func callBase() {
+        let _ = baseMethod() 
+    }
+}
+
+InheritedMemberTestSuite.test("Look up base methods from extensions") {
+  let a = Bar.Derived()
+  a.callBase()
+}
+
 runAllTests()