clang-format: [JS] wrap optional properties in type aliases.

Summary:
clang-format wraps object literal keys in an object literal if they are
marked as `TT_SelectorName`s and/or the colon is marked as
`TT_DictLiteral`. Previously, clang-format would accidentally work
because colons in type aliases were marked as `TT_DictLiteral`. r310367
fixed this to assing `TT_JsTypeColon`, which broke wrapping in certain
situations. However the root cause was that clang-format incorrectly
didn't skip questionmarks when detecting selector name.

This change fixes both locations to (1) assign `TT_SelectorName` and (2)
treat `TT_JsTypeColon` like `TT_DictLiteral`.

Previously:

    type X = {
      a: string, b?: string,
    };

Now:

    type X = {
      a: string,
      b?: string,
    };

Reviewers: djasper, sammccall

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D36684

llvm-svn: 310852