SplitEmptyFunction should be true in the Mozilla coding style

Summary:
As defined here: https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Coding_Style#Classes
See for the downstream bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1399359

Reviewers: Typz, djasper

Reviewed By: Typz

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D37795

llvm-svn: 313182