clang-format: [JS] don't break comments before any '{'

Summary:
Previously, clang-format would avoid breaking before the first `{`
found, but then happily break before subsequent '{'s on the line. This
change fixes that by looking for the first location that has no opening
curly, if any.

Reviewers: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50230

llvm-svn: 338837