[clang-format] Fix MatchingOpeningBlockLineIndex computation
Summary:
Computed line index must be relative to the current 'parent' node, and
thus use CurrentLines instead of Lines.
Without this, a child line's MatchingOpeningBlockLineIndex is out of
range of the parent's list of line, which can cause crash or unexpected
behavior if this field is used in childs.
Contributed by @Typz!
Reviewers: krasimir, djasper
Reviewed By: krasimir
Subscribers: cfe-commits, klimek
Differential Revision: https://reviews.llvm.org/D32524
llvm-svn: 303353