clang-format: [JS] test declared fields.

Summary:
TypeScript now supports declaring fields:

    class Foo {
      declare field: string;
    }

clang-format happens to already format this fine, so this change just
adds a regression test.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69972

Author: mprobst

clang/unittests/Format/FormatTestJS.cpp

@@ -2369,5 +2369,12 @@ TEST_F(FormatTestJS, SupportPrivateFieldsAndMethods) {
                "  static #staticPrivateMethod() {}\n");
 }
 
+TEST_F(FormatTestJS, DeclaredFields) {
+  verifyFormat("class Example {\n"
+               "  declare pub: string;\n"
+               "  declare private priv: string;\n"
+               "}\n");
+}
+
 } // namespace format
 } // end namespace clang