clang-format: [JS] handle semis in generic types.

Summary:
TypeScript generic type arguments can contain object (literal) types,
which in turn can contain semicolons:

    const x: Array<{a: number; b: string;} = [];

Previously, clang-format would incorrectly categorize the braced list as
a block and terminate the line at the openening `{`, and then format the
entire expression badly.

With this change, clang-format recognizes `<` preceding a `{` as
introducing a type expression. In JS, `<` comparison with an object
literal can never be true, so the chance of introducing false positives
here is very low.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D40424

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@318975 91177308-0d34-0410-b5e6-96231b3b80d8