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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290959 91177308-0d34-0410-b5e6-96231b3b80d8