Improved non-namespace overriding

Per @ahejlsberg's suggestion, only overwrite a namespace
`valueDeclaration` if the new declaration is not a namespace itself. This
means that if there are multiple namespace declarations, and nothing else,
`valueDeclaration` will be the first namespace declaration, not the last.

Author: sandersn

src/compiler/binder.ts

@@ -131,7 +131,8 @@ namespace ts {
             }
 
             if (symbolFlags & SymbolFlags.Value &&
-                (!symbol.valueDeclaration || symbol.valueDeclaration.kind === SyntaxKind.ModuleDeclaration)) {
+                (!symbol.valueDeclaration ||
+                 (symbol.valueDeclaration.kind === SyntaxKind.ModuleDeclaration && node.kind !== SyntaxKind.ModuleDeclaration))) {
                 // other kinds of value declarations take precedence over modules
                 symbol.valueDeclaration = node;
             }

src/compiler/checker.ts

@@ -294,7 +294,8 @@ namespace ts {
                 }
                 target.flags |= source.flags;
                 if (source.valueDeclaration &&
-                    (!target.valueDeclaration || target.valueDeclaration.kind === SyntaxKind.ModuleDeclaration)) {
+                    (!target.valueDeclaration ||
+                     (target.valueDeclaration.kind === SyntaxKind.ModuleDeclaration && source.valueDeclaration.kind !== SyntaxKind.ModuleDeclaration))) {
                     // other kinds of value declarations take precedence over modules
                     target.valueDeclaration = source.valueDeclaration;
                 }