Clang importer: filter out declarations found in the Standard Library.

With this, everything builds and runs properly when the Swift name
lookup tables are enabled for AnyObject and module-scope lookups.

Author: DougGregor

lib/ClangImporter/ClangImporter.cpp

@@ -4559,6 +4559,13 @@ void ClangImporter::Implementation::lookupValue(
       if (!decl) continue;
     }
 
+    // If we found a declaration from the standard library, make sure
+    // it does not show up in the lookup results for the imported
+    // module.
+    if (decl->getDeclContext()->isModuleScopeContext() &&
+        decl->getModuleContext() == getStdlibModule())
+      continue;
+
     // If the name matched, report this result.
     if (decl->getFullName().matchesRef(name))
       consumer.foundDecl(decl, DeclVisibilityKind::VisibleAtTopLevel);

test/IRGen/existentials_objc.sil

@@ -1,6 +1,7 @@
 // RUN: rm -rf %t && mkdir %t
 // RUN: %build-irgen-test-overlays
 // RUN: %target-swift-frontend -sdk %S/Inputs -I %t %s -emit-ir | FileCheck %s
+// RUN: %target-swift-frontend -enable-swift-name-lookup-tables -sdk %S/Inputs -I %t %s -emit-ir | FileCheck %s
 
 // REQUIRES: CPU=x86_64
 // REQUIRES: objc_interop