clang-format: [JS] parse async function declarations.

Summary:
Previously, clang-format would accidentally parse an async function
declaration as a function expression, and thus not insert an unwrapped
line for async functions, causing subsequent functions to run into the
function:

    async function f() {
      x();
    } function g() { ...

With this change, async functions get parsed as top level function
declarations and get their own unwrapped line context.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D32590

llvm-svn: 301538