[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