[Index] Remove reference to `UnresolvedUsingIfExists`

[bnbarham]

Summary

Assuming ns::foo doesn't exist, given:

using ns::foo __attribute__((using_if_exists));

The AST will look something like:
UsingDecl

UsingShadowDecl
  UnresolvedUsingIfExistsDecl

Thus we end up adding a reference to UnresolvedUsingIfExistsDecl when
processing UsingDecl, but never add the decl itself. In this case the
decl is really the UsingDecl anyway though (which we do output), so it
makes more sense to just remove the extra reference.

[bnbarham]

clang/test/Index/using_if_exists.cpp

10

The AST here is:
UsingDecl -> UsingShadowDecl -> UnresolvedUsingIfExistsDecl

So the UnresolvedUsingIfExistsDecl is a reference, which is a bit weird since there is no decl. But... there is no decl ("foo" doesn't exist), so this seems somewhat reasonable. Any objections?

[bnbarham]

After speaking with Ben, we decided it makes more sense to just remove the reference entirely.

[bnbarham]

Update title/message

[benlangmuir]

Minor suggestion for the test case, but otherwise LGTM.

clang/test/Index/using_if_exists.cpp

10

Could we drop the [[@LINE-2]]:11 | part of the CHECK-NOT to make it a bit more robust?

[bnbarham]

Remove line for <unknown> check

clang/lib/Index/IndexDecl.cpp

public:
 
   bool VisitUsingDecl(const UsingDecl *D) {
     IndexCtx.handleDecl(D);
 
     const DeclContext *DC = D->getDeclContext()->getRedeclContext();
     const NamedDecl *Parent = dyn_cast<NamedDecl>(DC);
     IndexCtx.indexNestedNameSpecifierLoc(D->getQualifierLoc(), Parent,
                                          D->getLexicalDeclContext());
-    for (const auto *I : D->shadows())
+    for (const auto *I : D->shadows()) {
+      // Skip unresolved using decls - we already have a decl for the using
+      // itself, so there's not much point adding another decl or reference to
+      // refer to the same location.
+      if (isa<UnresolvedUsingIfExistsDecl>(I->getUnderlyingDecl()))
+        continue;
+
       IndexCtx.handleReference(I->getUnderlyingDecl(), D->getLocation(), Parent,
                                D->getLexicalDeclContext(), SymbolRoleSet());
+    }
     return true;
   }
 
   bool VisitUsingDirectiveDecl(const UsingDirectiveDecl *D) {
     const DeclContext *DC = D->getDeclContext()->getRedeclContext();
     const NamedDecl *Parent = dyn_cast<NamedDecl>(DC);
 
     // NNS for the local 'using namespace' directives is visited by the body

clang/test/Index/using_if_exists.cpp

+// RUN: c-index-test core -print-source-symbols -- %s -target x86_64-unknown-unknown 2>&1 | FileCheck %s
+
+namespace ns {
+//  void foo();
+}
+
+using ns::foo __attribute__((using_if_exists));
+// CHECK: [[@LINE-1]]:11 | using/C++ | foo | c:@UD@foo | <no-cgname> | Decl | rel: 0
+// CHECK-NOT: <unknown>