Allow @desc to be treated as a type annotation at .i.js generation time.

This means that the pattern:
    /** @desc desc */
    const MSG_NAME = goog.getMsg('desc');
will be inferred to have string type, rather than requiring an explicit
type annotation.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171081663

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

@@ -365,13 +365,21 @@ JSDocInfo getJsDoc() {
       return NodeUtil.getBestJSDocInfo(lhs);
     }
 
+    /**
+     * Remove this "potential declaration" completely.
+     * Usually, this is because the same symbol has already been declared in this file.
+     */
     void remove(AbstractCompiler compiler) {
       Node statement = getStatement();
       NodeUtil.deleteNode(statement, compiler);
       statement.removeChildren();
     }
 
-    void maybeRemoveRhs(AbstractCompiler compiler) {
+    /**
+     * Simplify this declaration to only include what's necessary for typing.
+     * Usually, this means removing the RHS and leaving a type annotation.
+     */
+    void simplify(AbstractCompiler compiler) {
       Node nameNode = lhs;
       JSDocInfo jsdoc = getJsDoc();
       if (jsdoc != null && jsdoc.hasEnumParameterType()) {
@@ -503,7 +511,7 @@ private void processName(PotentialDeclaration decl) {
           }
           break;
         case REMOVE_RHS:
-          decl.maybeRemoveRhs(compiler);
+          decl.simplify(compiler);
           break;
         case REMOVE_ALL:
           decl.remove(compiler);
@@ -748,6 +756,9 @@ private static class JsdocUtil {
     private static JSDocInfo pullJsdocTypeFromAst(
         AbstractCompiler compiler, JSDocInfo oldJSDoc, Node nameNode) {
       checkArgument(nameNode.isQualifiedName());
+      if (oldJSDoc != null && oldJSDoc.getDescription() != null) {
+        return getConstJSDoc(oldJSDoc, "string");
+      }
       if (!nameNode.isFromExterns() && !isPrivate(oldJSDoc)) {
         compiler.report(JSError.make(nameNode, CONSTANT_WITHOUT_EXPLICIT_TYPE));
       }

test/com/google/javascript/jscomp/ConvertToTypedInterfaceTest.java

@@ -1076,4 +1076,20 @@ public void testGoogScopeLeftoversAreRemoved() {
             "",
             "a.b.c.d.e.f.g.Foo = class {};"));
   }
+
+  public void testDescAnnotationCountsAsTyped() {
+    test(
+        LINE_JOINER.join(
+            "goog.module('a.b.c');",
+            "",
+            "/** @desc Some description */",
+            "exports.MSG_DESCRIPTION = goog.getMsg('Text');",
+            ""),
+        LINE_JOINER.join(
+            "goog.module('a.b.c');",
+            "",
+            "/** @const {string} @desc Some description */",
+            "exports.MSG_DESCRIPTION;",
+            ""));
+  }
 }