clang-format: [JS] handle union types in arrow functions.

Summary: clang-format would previously fail to detect that an arrow functions parameter block is not an expression, and thus insert whitespace around the `|` and `&` type operators in it.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D36147

llvm-svn: 309707