Improve valueDeclaration for js module merges (#24707)

Nearly everything in a merge of JS special assignments looks like a
valueDeclaration. This commit ensures that intermediate "module
declarations" are not used when a better valueDeclaration is available:

```js
// File1.js
var X = {}
X.Y.Z = class { }

// File2.js
X.Y = {}
```

In the above example, the `Y` in `X.Y.Z = class { }` was used as the
valueDeclaration for `Y` because it appeared before `X.Y = {}` in the
compilation.

This change exposed a bug in binding, #24703, that required a change in
typeFromPropertyAssignmentOutOfOrder. The test still fails for the
original reason it was created, and the new bug #24703 contains a repro.

src/compiler/binder.ts

@@ -236,7 +236,7 @@ namespace ts {
             if (symbolFlags & SymbolFlags.Value) {
                 const { valueDeclaration } = symbol;
                 if (!valueDeclaration ||
-                    (valueDeclaration.kind !== node.kind && valueDeclaration.kind === SyntaxKind.ModuleDeclaration)) {
+                    (valueDeclaration.kind !== node.kind && isEffectiveModuleDeclaration(valueDeclaration))) {
                     // other kinds of value declarations take precedence over modules
                     symbol.valueDeclaration = node;
                 }

src/compiler/checker.ts

@@ -921,7 +921,7 @@ namespace ts {
                 target.flags |= source.flags;
                 if (source.valueDeclaration &&
                     (!target.valueDeclaration ||
-                        (target.valueDeclaration.kind === SyntaxKind.ModuleDeclaration && source.valueDeclaration.kind !== SyntaxKind.ModuleDeclaration))) {
+                     isEffectiveModuleDeclaration(target.valueDeclaration) && !isEffectiveModuleDeclaration(source.valueDeclaration))) {
                     // other kinds of value declarations take precedence over modules
                     target.valueDeclaration = source.valueDeclaration;
                 }

src/compiler/utilities.ts

@@ -451,6 +451,16 @@ namespace ts {
         return isModuleDeclaration(node) && isStringLiteral(node.name);
     }
 
+    /**
+     * An effective module (namespace) declaration is either
+     * 1. An actual declaration: namespace X { ... }
+     * 2. A Javascript declaration, which is:
+     *    An identifier in a nested property access expression: Y in `X.Y.Z = { ... }`
+     */
+    export function isEffectiveModuleDeclaration(node: Node) {
+        return isModuleDeclaration(node) || isIdentifier(node);
+    }
+
     /** Given a symbol for a module, checks that it is a shorthand ambient module. */
     export function isShorthandAmbientModuleSymbol(moduleSymbol: Symbol): boolean {
         return isShorthandAmbientModule(moduleSymbol.valueDeclaration);