Recognize = as equivalent to ? in JSDoc signatures

Fixes #6811

src/compiler/parser.ts

@@ -5727,6 +5727,9 @@ namespace ts {
             function parseJSDocParameter(): ParameterDeclaration {
                 const parameter = <ParameterDeclaration>createNode(SyntaxKind.Parameter);
                 parameter.type = parseJSDocType();
+                if (parseOptional(SyntaxKind.EqualsToken)) {
+                    parameter.questionToken = createNode(SyntaxKind.QuestionToken);
+                }
                 return finishNode(parameter);
             }