clang-format: [JS] avoid indent after ambient function declarations.

Summary:
Before:
  declare function foo();
    let x = 1;

After:
  declare function foo();
  let x = 1;

The problem was that clang-format would unconditionally try to parse a child block, even though ambient function declarations do not have a body (similar to forward declarations).

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D28246

llvm-svn: 290959