BuildGlobalNamespace will not create duplicate Refs

Previously it was up to the caller of scanNewNodes() to avoid passing
BuildGlobalNamespace nodes that might already have references.
This has led to exceptions being thrown because multiple references
were created for the same Node.

This change makes BuildGlobalNamespace and the Name class it depends on smart
enough to avoid creation of additional Refs for the same Node even when
scanNewNodes() is passed AST nodes that already have Refs.

Effectively this means that code modifying the AST can safely pass any Nodes
they think may need reconsideration to the scanNewNodes() method and rely on
BuildGlobalNamespace to do the right thing.

On the other hand, I've added several checks to restrict changing a Ref's Node.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=233448314

src/com/google/javascript/jscomp/AggressiveInlineAliases.java

@@ -565,8 +565,8 @@ private void inlineGlobalAliasIfPossible(Name name, Ref alias, GlobalNamespace n
           checkState(aliasParent.isAssign(), aliasParent);
           Node aliasGrandparent = aliasParent.getParent();
           aliasParent.replaceWith(alias.getNode().detach());
-          aliasingName.removeRef(aliasDeclaration);
-          aliasingName.removeRef(aliasDeclaration.getTwin());
+          // remove both of the refs
+          aliasingName.removeTwinRefs(aliasDeclaration);
           newNodes.add(new AstChange(alias.module, alias.scope, alias.getNode()));
           compiler.reportChangeToEnclosingScope(aliasGrandparent);
         } else {