clang-format: [JS] Fix line breaks before comments when sorting imports.

Summary:
Previously, clang-format would always insert an additional line break after the
import block if the main body started with a comment, due to loosing track of
the first non-import line.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D24708

llvm-svn: 281888