clang-format: [JS] conditional types.

Summary:
This change adds some rudimentary support for conditional types.
Specifically it avoids breaking before `extends` and `infer` keywords,
which are subject to Automatic Semicolon Insertion, so breaking before
them creates incorrect syntax.

The actual formatting of the type expression is odd, but there is as of
yet no clear idea on how to format these.

See https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#conditional-types.

Reviewers: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52536

llvm-svn: 343179