clang-format: introduce InlineOnly short function style

Summary:
This is the same as Inline, except it does not imply all empty
functions are merged: with this style, empty functions are merged only
if they also match the 'inline' criteria (i.e. defined in a class).

This is helpful to avoid inlining functions in implementations files.

Reviewers: djasper, krasimir

Reviewed By: djasper

Subscribers: klimek, rengolin, cfe-commits

Differential Revision: https://reviews.llvm.org/D34399

llvm-svn: 305912